If you want backwards compatibility for the PS3, go into your local Gamestop and ask if they have any launch PS3's. Ask for the 60 GB PS3. Thats what my buddy did, and he still plays his old PS2 games on it. Also any PS3 will still play PS1 games.
Here are the genre breakdowns. If you're only interested in one particular genre, then go with that console.
Shooters-360. And it's really not that close. Depends how Killzone 3 and Resistance 3 do next year when they come out for the PS3. COD runs better on the 360, Halo is still popular. Superior online capabilities for the 360 gives them the nod here.
Action/Adventure-PS3. Aside from the multiplatform games on each system (toss up), the PS3 has the Uncharted series, which is the #1 series out. I'm the biggest Metal Gear Solid fan, but the Uncharted series blows it out of the water. It's a console seller by itself. Metal Gear Solid 4 is quite amazing though. So is God of War 3.
RPG-PS3. Sony has always been about RPGs, Demons Souls (best RPG on any console), Valkyria Chronicles, Persona.
Sports-PS3. This would be dead even if it wasn't for MLB The Show. If you don't like baseball, then it's a toss up. All other games are even. The Show pushes the PS3 over the top. Dwarfs the 2K series.
Racing-PS3. Again, like sports, its only because of one game. Gran Turismo 5. Everything else is even, like Need for Speed.
Music- 360. Because of the online capabilities, the 360 is the way to go if you're a big Rock Band player. Downloading songs, connecting with friends to play. 360 is the way to go.
In terms of exclusives, each console has their stars.
360- Halo series, Gears of War series, Fable series, Forza series, Left 4 Dead series.
PS3- Resistance series, Killzone series, Uncharted series, Gran Turismo series, LittleBigPlanet series, God of War series.
The console exclusive pool is growing smaller though. PS3 lost Final Fantasy last year, and Metal Gear Solid too, since Rising will be on the 360 (though its more of a spin off from the series). 360 lost Dead Rising and Mass Effect, and Halo is done, Bungie is no longer making exclusive games.
If you've always been a PlayStation guy, there's no need to change now, just get backwards compatible PS3 from Gamestop. Easy as that. If you do get a PS3, Uncharted 1 and 2 need to be your first two purchases. Without a doubt, the best video game series right now. Uncharted 3 coming November 1, 2011.