I've played FO3 since release, pretty much, for many, many hours. Usually, I create one character and stick to that forever, but in FO3, after things got boring with the first playthrough, I did a second one for the first time in all of my RPG history.
Fallout Wanderer's Edition (FWE) on hardcore settings, Feng Shui to decorate and tweak my Megaton house just a bit [containers, mainly] and many mods more are what defined, and still does define, that second playthrough. And I'm still not tired of it. Only reason I stopped playing is I've never played New Vegas before, which is what I'm doing now. Over 100h as of now, modded in mostly the same way as FO3.
To me, somehow, NV seems so much smaller. The map / world, I mean. I feel like I can walk for a long while with not much purpose, and it's all plain and empty - desert, of course. In FO3, there was much more to see in the same amount of space, and FO3 seems so much bigger to me, in terms of map size and stuff on it. Might be just a feeling, maybe they're identical in pure dimensions, but NV sure felt weirdly empty my first time around.
I like both games. As for which I'll end up liking better, we'll see. The casino games are a bonus, for sure - but then again, there are other annoying things tied to those, too and I barely ever enter the casinos because of that.
Well, bottom line: In all Bethesda Games, Fallout or Elder Scrolls, do not, I repeat, do NOT turn your back to mods. There's some ridiculously awesome shiz out there that will improve your game of choice.
... and I loved the STALKER games to death, too. If you've got them all, I recommend checking out STALKER Call of Chernobyl, which fuses all the maps together.