I think I'll have to go with an HD console for this one. I've always loved Nintendo's 1st party offerings, but after the first year of the generation I've had this feeling that they've tried to "casualize" their franchises for the greater good of gaming or something along those lines. I was excited for Super Smash Bros. Brawl and Mario Kart Wii... Now I'm scared to see the next F-Zero and Star Fox (if they're not handled by third parties like the previous iterations), heck even Zelda! I must say I really liked Super Mario Galaxy and Metroid Prime 3: Corruption though.
Now, I'm picking the HD consoles because of one thing mostly: XBLA and PSN are two awesome features to come out of this generation. Developers can release remakes of their old games so new gamers get to enjoy the goodness of the past, or they can even release ports of their original games with some updates added to them (online multiplayer for example). Even entirely new games like Shadow Complex, Castle Crashers and Fat Princess can come out of these services. Wiiware is neat as well, but it's mostly filled with shovelware since you can't really have "proper" lengthy games on the system because of the memory limit Nintendo has put on the developers. Hopefully, by next-gen, Nintendo will have proper online services and keep up with the competition because it's inevitable that this is the way gaming is going to go from now on.
Anyways... with all that said, I have to go with the 360's game library as well. I'm more familiarized with the IPs already available on the system like Halo (which I find incredibly fun and never gets old), Banjo-Kazooie, etc.. New IPs like Gears of War 1/2, Mass Effect (at the time it was exclusive), and many more have blown my mind. The other 3rd party games that came out for the system have been great so far as well (for the 360/PS3/PC obviously).
I must say that it was hard to choose between the PS3's game library and the 360's game library because of titles like Uncharted 1/2, Valkyria Chronicles and Metal Gear Solid 4. I guess one console has more single player game experiences to offer than the other, but it's the "party system" that will always get my vote. 
Random game thought :
Why is Bionic Commando Rearmed 2 getting so much hate? We finally get a real game and they're not even satisfied... I'm starting to hate the gaming community so f****** much...







