I own a 40gb PS3 (I really don't see what's the big fuss about backwards compatibility... if you have PS2 games, you have a PS2, just use it.) and I am very very satisfied by it.
I'd consider myself a very casual gamer, too. I tought I would go for the Wii but the remotes seemed gimmicky and most of the games for it are not my style so I had to choose between a 360 and a PS3
I bought a 360 at first because I wanted to mod it and download/copy games. My 360 died in 2 weeks, I got a refund and bought the PS3 instead. I do miss a couple of titles (Gears of War and other COOP FPS come to mind) but the PS3 has a rapidly growing list of very fun titles to make up for it. The PS3 OS is such a charm, I love it so dearly! I honestly did not think I would enjoy the media center possibilities so much but I find myself using them everyday to watch downloaded episodes of LOST or movies or music. Internet playing is free, which is great because I don't play much online and I would hate to have to spend money for the 2-3 online games per month I play. The remote controls are rechargeable, which is also a nice detail. The blue-ray reader is really fun, but wasn't a selling point for me..
I don't want to tell you to buy one or the other, that's really up to you... and if you have friends who have either, it's a good idea to get the same console as your friends, in order to trade/borrow games and controllers.
I guess I just wanted to point out that I'm also a quite casual gamer and I never regretted my choice of the PS3. Both consoles do make excellent picks, though. Since I bought mine, 3 of my friends bought PS3 (both had a Wii before and weren't using it much) because they were jealous.
EDIT: Just to clear things up a bit, if I had to make the choice again, I would opt for the PS3 again because of the better reliability, blue-ray player, free online, better OS, upcoming exclusives and because it comes with more stuff bundled (like wireless, HDMI cable, and the controllers dont need batteries, etc.)