If your PS3 has PS2 compatibility and if you have a good PC then why not a GameCube?
Most games on the Xbox are on other consoles or the PC and the ones that aren't are most likely already Xbox 360 compatible. If I remember correctly, I once made a list of games that are only on the Xbox and only 20 were exclusive for it. The Gamecube has a lot of exclusives and plenty of games to chose from. Plus, if you got a Wii, all the games would run on it.

Thanks guys, I went with the PS2. You're all so helpful... *sniff*

I ordered a slim Ceramic White PS2 and I'm thinking of getting the Swap Magic 3.6 for... uh... backups.

Anyway, here are just some reasons to why I chose a PS2.
First, the catalog. It's much larger than the Xbox, and it's not discontinued and given the catalog of upcoming games, it won't be anytime soon. I've also got a thing for JRPGs, so that's always nice.
Secondly, my PS3 doesn't do very good BC (even the launch PS3s had trouble with sound on some PS2 games) so a PS2 was a good choice in complementing the system.
The Xbox has an advantage, which is that it can be used as a media center. And Aliasalpha made the excellent yet subtle point of a way to hide my hentai tentacle porn, which made it even better. But I have a freaking computer for that, as well as a DVR.
The PS2 is also goddamn cheap! I bought mine off Play Asia (free shipping in Hong Kong, dudes) because I live in Hong Kong and it's pretty cheap. I had a coupon, so I just used that and knocked off a fair bit of money in the total. I ordered a white NTSC-J (native to Asia) PS2, added a 8MB memory card (might get those mystical 128MB storage units if I use this more often), got my hands on a second controller in black, and bought The Godfather, Lego Star Wars 2, and I bought ICO retail today. I've planned to get the Swap Magic and the Swap Tool Pro later.

"The Xbox has an advantage, which is that it can be used as a media center."
Actually a modded PS 2 can be a media center too. When I had PS2 I was using SMS player software. It could play avi, mpeg, mp3 files from disks, usb storage or on network. And this was about 3 years ago. So I think now in new versions of this software there are some new formats. Also there are NES and Sega emulators for PS2 if you are interested.

Interesting, but it needs a USB/network thing. I'd much rather have it from hard drive... which doesn't exist anymore.

Also wasn't PS2 modding all chip based? Xbox's before I think revision 1.5 could be modded by a well timed swap of the hard drive cable and a bit of PC boot disc tomfoolery
What would be ideal is if there was a reliable PS2 emulator for the xbox

I am not a console user but I prefer PS2. It has huge great games archive.

No, softmodding on the PS2 lets you switch discs around without any warranty voiding.

Xbox, since your PS3 should play PS2 games.
But Xbox anyway.

The PS2 has Trapt, Odin Sphere, MGS 2/3, Yakuza 2, and The Grim Reaper Has A Flaming Motorcycle: Dante Is In This Game (along with the spinoff series of The Grim Reaper Has A Flaming Motorcycle, High-School Students Fight Demons With The Power Of Friendship). Need I say more?

Forgot about Tekken, Gran Turismo, God of War etc.

Are those even real games?

Yes.
The Grim Reaper Has A Flaming Motorcycle 3 on Wikipedia
[url=http://en.wikipedia.org/wiki/Shin_Megami_Tensei:_Persona_3]High-School Students Fight Demons With The Power Of Friendship 3 on Wikipedia[/url]