PS3 online is now really similar to 360. As of firmware 2.41, you can bring up the Cross Media Bar in-game, and there are also trophies just like Xbox Live Achievements. There are smaller things that the PS3 lacks (e.g. you can't leave voicemail for people on your friends list), but going forward they're going to be very similar. All of the best games have already been patched for rumble, and all games going forward will have achievements.
However, Xbox Live is the slickest thing out there. It's perfect integrated into every game and has an amazing feature set. I don't have anything bad to say about it, and it'll only get better when they revamp it in the fall.
Downloadable games (meaning non-retail) from each service are pretty equal, but the 360 has a cooler lineup IMHO (Ikaruga and Rez HD beat out Super Stardust HD, despite my intense love for SSHD):
360 - Ikaruga, Pac-Man Championship Edition, Galaga Legions, Rez HD, Geometry Wars 1 & 2, Soulcalibur, Undertow
PS3 - Super Stardust HD, Echochrome, Tekken 5, Calling all Cars, Everyday Shooter
In terms of downloadable video content, I think that Sony now has the slight edge here. You can rent HD/SD movies/TV shows on Xbox Live and stream them (and those with a Netflix account can stream movies from their streaming queue), but you can't purchase them (some 360's don't have a hard drive to save them to). On the PS3 you can rent HD/SD movies/TV shows, or purchase them and keep them on your hard drive. And of course, you can also rent/purchase them on Blu-Ray disc as well.
Big 360 Exclusives: Halo 3, Gears of War, Gears of War 2 (holiday), Ninja Gaiden II, Crackdown, Mass Effect, Fable 2 (holiday), Forza Motorsport 2
Big PS3 Exclusives: Metal Gear Solid 4, LittleBigPlanet (holiday), Gran Turismo 5 Prologue, Uncharted: Drake's Fortune, Ninja Gaiden Sigma, Motorstorm 1 & 2, Resistance 1 & 2, MLB 08 The Show, Killzone 2 (February 2009)
Games that are better on 360 than PS3 due to sloppy porting - Madden 08, Orange Box, Assassin's Creed, Skate, Lost Planet
Games that are good on both: CoD4, Burnout Paradise, Rock Band, Guitar Hero, BioShock
Games that are good on both, but only 360 has downloadable content: Grand Theft Auto IV, Elder Scrolls: Oblivion, Fallout 3
Others:
- Virtua Fighter 5 - 360 version has online play, PS3 doesn't
- Unreal Tournament III - PS3 version allows you to use user-created maps
A big factor is how many of your friends have each system, too. That's huge. There's no point in being the only one in your crew with a PS3 if all of your buddies have 360's.
My thoughts on each platform...
PS3: Super sexy hardware, made for HD home theater, the best games you've never heard of, online is right there
360: All your buddies probably have one, great game selection, excellent online service
I think at this point people buy PS3 for the home theater stuff first, game selection second. They buy the 360 for the game selection first, the home theater stuff second.