Really it depends on which games you want to play. Very few games are platform exclusive these days, so take a hard long look at which games ARE exclusive and decide which ones you want to play. Weigh them out, and there is your answer!

Personally, if I absolutely had to choose one or the other, I'd go with the PS3. The reason is that several big-name games I care about are coming out for it (Final Fantasy, MGS, GTA4 and some others.) Also, the 360 has lots of great games for it, but few games that I am as excited about as, say, Metal Gear Solid. Plus, I have less time to play games these days, what with work and all. So I would rather spend my limited gaming time on the few titles I really care about, than the tons of 360 titles which are plenty good.

Of course, I don't regret getting a 360 at all, but the only games I've spent a significant amount of time on are Halo 3, Mass Effect, and Rock band.

I might end up buying a PS3 at some point for the blu ray option. I also like the look of some of the new PS3 releases coming out...

PS3 games are looking better and better all the time, plus it has some GREAT exclusives on the horizon.

Well since the competition is getting heated now between the PS3 and 360, it's a pretty hard choice. Here's something to consider...

Future Xbox 360 exclusives:
Gears of War 2
Ninja Gaiden 2
Fable 2
Bioshock 2
GTA IV Exclusive content
Alan Wake
Dead or Alive 5
Halo Wars
Tales of Vesperia

Present Xbox 360 Exclusives
Halo 3
Gears of War
Bioshock
Mass Effect
Dead or Alive 4
Blue Dragon
Crackdown
Lost Odyssey
Dead Rising
PGR 3 & 4
Saints Row

Future Playstation 3 Exclusives
Killzone 2
Final Fantasy 13
Metal Gear Solid 4
Haze
Gran Turismo 5
LittleBigPlanet
Tekken 6
Disgaea 3
Star Ocean 4
Echochrome
God of War 3
Resistance 2

Present Playstation 3 Exclusives
Ratchet and Clank Future: Tools of Destruction
Heavenly Sword
Lair
Time Crisis 4
Folklore
Ninja Gaiden Sigma
Resistance: Fall of Man
Uncharted: Drake's Fortune

Right now, the Xbox 360 has the better game line up. I'd say the Playstation 3 has about 6 good exclusives out right now. The Xbox 360 has about twice that amount(that are good).
However, if you look at the future lineup of the Playstation 3, there's tons of great games coming. Almost all of which are bound to be good. The same can go for the Xbox 360 as well, it doesn't seem to be slowing down. I see them both doing equally well in the future.
As for high definition, both systems are about the same. If you're playing Assassin's Creed or Devil May Cry 4 on your TV, you won't notice the difference between the Xbox 360 and the PS3 version. If I were to pick, I would go with an Xbox 360, but ONLY because of the online features. The 360 almost completely obliterates the PS3 when comparing downloadable content and online play in general. That's something it has a leg up on, and it's most likely going to stay like that. Even with the PS3's Home, I doubt it can catch up now. But if online isn't an issue for you, the PS3 is great.
edit: geez long post.

Also note Metal Gear Solid 4 is like the most important game in the world so...

I have both a Wii and a 360. I got the Wii on launch, played Zelda, then basically let the sucker sit and collect dust for a year. Got my 360, a few games, and never looked back. I haven't used a console that much since the bloody PS1.

That said, I haven't had much experience with a PS3; I had one for about a month or so, back in January, and the games lineup didn't really interest me that much. Basically, I don't regret buying my 360 a bit, and I would go for a 360, personally, but hey, I'm probably biased. *shrug* Best to ask someone with a more valid opinion.

New Zealand represent!! haha. I would go with a 360, although I have a Playstation 3(Weird aye?) I'm starting to regret trading in at launch without even looking at the lineup. Just listen to what Neophyte said, he's pretty on to it haha, except Lair, that's a BAD game.

I have both consoles and I love both dearly however I play 360 much more often than PS3.  Since I generally prefer Japanese games (expecially jRPGs) this really says something about how weak the PS3 really is right now.  However I am confident that it will be a fine console in the near future because its major releases are going to start trickling out.  The PS3 launch in general was just a complete disaster and all of the big-name games were never scheduled for release in 2007 anyway. 

So, just like everyone else will say, if you prefer US-developed games, the 360 is a better choice, and if you prefer Japanese-developed games, the PS3 is probably a better choice.

You're gonna miss out on a lot of good games not matter which one you get. Personally I'd probably go for 360.

I'd go for the 360 just for the most awesome Xbox Live / XBLA.

the wii fucking sucks unless you are 10 or have friends over all the time, the 360 has no good games that aren't already available on PC, so i'd go with the ps3.

of course i can also think of other things to spend hundreds of dollars on but yeah.

Future Playstation 3 Exclusives
Killzone 2
Final Fantasy 13
Metal Gear Solid 4
Haze
Gran Turismo 5
LittleBigPlanet
Tekken 6
Disgaea 3
Star Ocean 4
Echochrome
God of War 3
Resistance 2

Present Playstation 3 Exclusives
Ratchet and Clank Future: Tools of Destruction
Heavenly Sword
Lair
Time Crisis 4
Folklore
Ninja Gaiden Sigma
Resistance: Fall of Man
Uncharted: Drake's Fortune

basically if this list raises anything other than revlusion kill yourself, snap, doop burn.

triples (GAME TERMS) aren't really worth it, get a 360 or a Wii.

the 360 has no good games that aren't already available on PC.

I can only count 3....
Bioshock
Gears of War
Mass Effect(soon)

That's only 3 games out of a library of 15+ good ones.

The future 360 titles don't compare to the future PS3 titles. Also, blu ray is the future.

I like my PS3 for the PSN, free internet, awesome games, familiar controller (that works really well), and best of all my voice chat. (Playstation Eye + PSN Chat = Vchat. Sweet!) The PS Eye itself grabs me, with that powerful microphone for online gaming. It's terrific for Rainbow Six and CoD4; even though the blutooth headset rocks my socks. Walking around my house and chatting freely is great, haha.
And, yeah, the line up coming in is nice. Littlebigplanet, PS Home, and stuff will keep my attention for quite a few years.

On a side note, my friend's friend has a 360, and friend has neither. He plays both equally, and when asked which he would buy if it came to it; he said angrily 'PS3', as if I'd insulted his intelligence by suggesting Microsoft.

Well I own a PS3, which instantly makes me support it. Sorry, but its the truth. The only two games I CARE about[even ][/even] are HAZE and MGS4. I don't understand why people think Haze will be a bomb but eh~

The Wii is a great console if you like to get drunk and party, but its never been a console that I'd personally choose to buy for single player use.

The 360 has the best library this is out right now, but it doesn't matter if you have a PC anyways, you'll only be missing HALO and a few others. However, it is the most expansive console right now, and if you don't want to spend all of your cash on a ballsy computer it's a good choice.

Still, I'd pick the PS3. Because I own one and every one flames it saying it SUCKS ASS but to be frank, I've never had any SUCK ASS problems with it and I've already totaled over 1,500$ in games for the machine so I figure it has to be good at SOMETHING.

The future 360 titles don't compare to the future PS3 titles. Also, blu ray is the future.

Wow, that is bad..