mystral: Robust swords do more damage, and sometimes have slightly better stats (amount of damage blocked, chance to inflict status effects, that sort of things). That's the only difference I think.
If you're wondering about crafting, I didn't find it necessary to craft steel swords during the game. You get good enough ones during the game either as loot or as quest rewards.
Crafting a good silver sword definitely came in handy in early chapter 2 though.
Senteria: Yeah I'm looking at the silver sword ones. Just need 5 more silver ore for it.
If you need silver ore, it's very rare to find it outside of blacksmith inventories.
However, if you leave the immediate area and come back, they'll have replenished their inventory. It's very easy (and not very expensive) to buy enough silver ore that way.
Actually, it's probably the only way to get enough silver ore to craft the decent silver swords, since there are no silver mines in-game.