Well I own wii, ps3, and 360. Here's the lowdown on all of them.
xbox 360- I have barley any games as of right now, not trying to sound trollish, but it's broken down on me 3 times now, so I get nervous using. I don't see why people complain about the noise, it's really not any louder than say a computer while gaming, it's just very constant. I mostly use it for demos, I love the marketplace it's updated very often, microsoft has done very well with this. Once bioshock drops I'm pickin that up.
wii- Loved this thing to death the first week I had it and now with lack of money, and lack of selection of games I've slightly lost interest. If you have one, I strongly suggest the game warioware, that's been my best experience so far, but for the time being I haven't used this one that often.
ps3- This one is the one I use the most for a few reasons. First off I'll state the system has NO games at the moment, thank god it's comin soon though, warhawk, lair, heavnly sword. But the reasons I've been using my ps3 the most is, one because the system is SOLID as a rock, I usually leave it on all day, or folding so I just grab my multi-plat games for that. But the main reason is bluray if you have and HDTV I strongly suggest getting into this, this is the real reason I prefer my ps3 at the moment, it's not games/graphics/online, it's the blu-ray
edit-
ahh forgot DS and psp
ds- use it a HELL of a lot more than my psp, I love all the non-games, I also love NSMB and all that fun stuff, it's just a great system, I get nervous hearing all the stories about them breaking, but I still play the hell out of it.
PSP- Haven't used this in a long time. Solid little system, great for multimedia, great graphics, an awesome interface, it's just a little boring right now. But GOW is on the way and hopefully some more after that.