Gog pro:

Mostly DRM free.
Plenty of sales & once in a while you'll get something cheaper here than steam (although legit key sites tend to be cheaper than steam too).
Offline installers (see cons...).
Once in a while gog connect will have more games added, so if own it already on steam, you may get it DRM free here for no fee.

Gog cons:

Minor;
Grim Dawn (one example) had something messed about on gog, in that you had to use galaxy to play co-op/online, but you can work around it with Tuungle etc.
There are competitions to win free games at times, but they're on Reddit/Twitter/FB, not (or rarely) actually on here for long time customers.
Some times games'll bug & show you galaxy achievements, even when you don't have galaxy installed.
Got a forum reply? Good luck noticing that "notification"

Serious;
Nagged, constantly, into "try galaxy".
The whole debacle of gog saying they're going to put galaxy installers, into the offline installers. I mean, seriously, wtf!?
Delayed time for patches with some games waiting months/years & still not getting them.
Some games getting new DLC which don't make it here.
Cross play only works with very few games, so you're less likely to find co-op/online parties. Worms W.M.D. when brand new, folk here couldn't find a single match at times.

Actual patches ~ Some games aren't given patches for offline installers, they're forced into doing full downloads any time there's a patch, even when that patch is only a few kb in size (Yes, I'm glaring at you Titan Quest...)

https://www.gog.com/forum/general/stahp_with_the_full_download_patches/page1

Even when the devs sent patches, gog can (& have) decided to remove them all & force a full download for offline installers. Asked on Grim Dawn forums, they'd stated they'd provided a patch, but that gog had decided to force full downloads instead.

Website doesn't tell you when you've had someone try to "friend" you, nor does it tell you if your games have new patches etc. Barefoot essentials is doing what the site should do.
Being told on the forums that "eventually" it may get fixed is just plain bad.

Steam pro:

Plenty of sales.
Patches are there as soon as they're available.
If you want to play co-op/online, there's a hell of a lot more options with steam.
A client that has years of testing put into it.
You can make money from steam cards, which you can even get from free games. Then you can buy more games, which technically cost you nothing.

Steam cons:

Need the client running for a lot of the games (but not all).
Support....lol.
Some games (like Magicka:Wizard Wars) you wouldn't be able to connect to Paradix servers, if steam was down for maintenance or whatever reason, meaning no play.

It's each to their own though.
Personally, I quit buying from steam last year (24 Sep, 2016 last purchase I paid for) but I also quit buying from gog too because of the state of things (Dec 6, 2016 last purchase I paid for) & now rely solely on legit key sites until they (both sites) get their acts together.

Homeworld Catac....erm, Emergence is now here & only here, but I'll have to live without it until I'm happy with how things here again, if ever, which will be easy as I've plenty of other games to play until (if) that happens ;)

Off Topic:

Your comment made my day & I really did laugh out loud.

(I had every single album of KISS including the hard to find "Originals" with the Army Stickers, Booklet, etc & gave it all away to my best friend because I had no way - at the time - to play it; and after selling my 1973 Cutlass Supreme “bondomobile,” I never had an 8-Track player again.)

GOG because they are consistent with what they offer (physical ownership for something that is sold like property)

If ever there comes a day where Steam offer a Netflix-like flat-rate to play a large selection of games (that don't suck), I'll consider that, but as long as they insist on selling games as goods, but with a remote service dependency and no worthwhile value (I don't give a damn about ladders, achievements or auto-updates) added for the restriction, I'll just feel ripped off buying from them.

Same here. I always do research before buying (I look at the score here, I look at the percentage of likes on Steam, I look at the aggregated critics score, I read at least 2 bad reviews), I never pre-order, I usually wait for good sales on games and if it isn't to my taste, I accept the loss.

I do at least like most games though which helps my batting average with this strategy being very good.

I can already imagine a post-apoc game when the main quest is to find bones82013 to get key piece of old tech to repair vault water supply : D

I mostly use Steam over GoG because on Steam I can purchase with card monies and because bundles give steam not gog keys. I haven't given any real money directly to Valve in years.

For me both.Buying games from both of the platforms is a lot easier than Origin and Uplay.That's why I love them both.

This could be a con as well: there have been more than a couple occasions where patches have broken something, and you can't opt out of them on Steam. At least on GOG, you can wait to see if the patch messes with anything before installing it.

Is it hard for some people to use both because "all my games in one place" or other reasons? Personally I'd encourage using GOG to support competition, but it's not like you can't use both. Both have their pros and cons.

I don't normally go 'vendor hunting' in my regular purchases and for my business, and I tend to stick with one supplier for convenience reasons and just because.

My link to gOg, then, goes back to, well, my join date and a bit earlier. I'd about given up on new gaming back in 2005 or so, with only a rare purchase here and there - maybe 1 or 2 per year. Just lost the excitement for the hobby, due to the games themselves at the time and also my own reasons. But I was still getting PC Gamer magazine back then, and they had mentioned gOg a few times here and there: how they were bringing back classics to play nice on newer systems and OSes, DRM-free, etc. You know the deal. Finally remembered to check them out and was pretty excited at the idea once I saw the store. Bought a few games and I was back into the hobby again.

I don't recall having checked out Steam at all up to that point, and didn't really have a reason to do so afterword. My only close call came with a brick-and-mortar purchase of a title that used the client.

Over the years I've read a bit about the differences between the two stores, and I feel I made the correct choice for me. I prefer the more hands-on client-free experience and that's what this gives me for now. I don't really need the much-larger catalog from the other store; I have less than 20% of the catalog here and I'll likely never have time to play half of my library.

Use both if you want, or use just one. I'm going to stick with this one unless and until they screw the pooch on meeting my customer service needs.

You can.
Just r'click to set patches to install only when you start game & play offline if wanting to play something that has a patch waiting.

While Steam is better for multiplayer games, I'd much rather have all my single-player games on GOG.

Yes, I realize that, but it's not really giving you the option of installing patches or not. It's a workaround that gives you the illusion of the option.

I remember downloading Steam as a kid. I played Team Fortress 2 and thought the Steam Overlay was the coolest thing ever. I dreamed of having a large game collection and tons of friends on Steam.

When I finally got those things several years later, I realized I didn't actually like the social aspect. I almost exclusively play single player games and I don't want people to bother me while I play. There are also times when I don't want people to know I'm playing a certain game, or when I don't want people to know that I'm online. Achievements are just plain stressful and developers rarely make interesting achievements anyway. Lastly, the "hours played" counter serves no purpose to me, I just feel guilty when I see the time on certain games. I'm very happy that GOG makes these features optional.

GOG is the closest approximation to the way I bought games as a kid - a personal experience, no peer pressure, just a CD of a game that looks cool. It's actually better than the old days thanks to reviews and lower prices - I got burned several times by $20 shovelware on the store shelf.

Also, these days it's impossible to just browse Steam. Everything is either AAA games everyone knows about, or Greenlight garbage. GOG's quality standards make it easier to just browse the store and find hidden gems.

The only advantages of Steam are their commitment to Linux and the ease of multiplayer. But again, I don't really play multiplayer.

I hate this mentality of some Steam users.

When faced with a flaw or issue with Steam, they come up with the convoluted work around and then act as if its perfectly normal and acceptable.

Me: The lock on the drivers side of my cars broken, need to get it fixed.
Steam User: Same thing happened to me, I just climb through the boot every time I use the car.
Me: Don't you want it fixed?
Steam USer: No, it was sold like that. Id rather use this clumsy work around rather than dare to think it should be fixed.

sorry.. what is the steam?