the ps3 has some of the best media capabilities you'll find anywhere (ie, a web browser which is great for viewing Freeones.com on your big screen TV, wi-fi built in, removable hard drive that is replaceable with pretty much any laptop hard drive, bluray player, plays a ton of video formats like Divx /Xvid, wmv, mp4, etc (the xbox does play some of these formats as well, like Divx, etc).
and one of the more important things is that the ps3 is known as a very reliable system that rarely breaks down. the xbox 360, on the other hand, is nortorious for breaking down and having problems with scratching discs ...though it's said that those problems aren't as common these days as it was a year or two ago.
if you have an earlier model of ps3, you can play all ps2 and ps1 games as well. that's one hell of a gaming library available on just one console. those systems are pretty hard to find now, tho. they've been discontinued and replaced with cheaper versions of the ps3 that don't have any PS2 compatibility (they still can play all ps1 games, tho), fewer usb ports, and no media card readers.
on the other hand ...the xbox is cheaper (the $199 price tag on the arcade unit looks great ...eventhough it doesn't come with a hard drive or wi-fi ...but it still looks great in most everyone's eyes versus the $399 tag of the ps3. the xbox also has a better online service, although you do pay extra for it.
another thing with xbox360 titles, is that the multiplatform titles (like Assassins Creed, for example) often times look slightly better or have slightly better framerates on the xbox than the ps3. this is usually because game developers had developed the game primarily for the xbox and then port it over to the ps3. the ps3 is also harder to program for and sometimes the end result is a somewhat sloppier version of the game) this problem isn't as common now as it was a year+ ago ...but still happens on occasion.
another thing the xbox has going for it are the "Achievements". every xbox game has these "rewards" implemented in the game ...but on the ps3, for the most part, there is nothing like that to be found. newer ps3 games have started to implement a "Trophy" system, which is basically the same as the xbox "Achievements", but few games have this feature. from what i hear, any game that's released this year onwards on the ps3 is required to have Trophy support ...so it should become quite common from here on out.
overall ...i think you get more bang for the buck (all things considered ...ie, bluray, etc) with the ps3 ...but looking at it purely from a gaming standpoint, which is the reason you're buying a gaming console in the first place, i'd say the xbox is a smoother ride (as long as it doesn't break down).
so, in short ...
ps3 = good gaming console with awesome media capabilities. if watching bluray movies on your hi-def TV is of interest to you and you'd like to play some games while your at it ...then ps3 is hard to top. it's online service is free, and works rather well on most occasions, but it's not quite as good as xbox live. it's game library is quite good now, too ...so there's no shortage of quality games. also, only certain games can be found on the playstation, too ...ie, God of War, Gran Turismo, Twisted Metal, Uncharted, Rachet and Clank, to name a few. These are all Sony's own titles and can't be found anywhere else.
xbox360 = if you already have a stand alone bluray player, or if bluray doesn't matter to you, and you're just looking for a pure gaming console (the xbox uses a standard DVD drive, so there's no Bluray) ...the xbox is hard to beat. it has a great online service (which does cost you money ...around $50 a year) and a slightly larger library of games due to it's year head start on Sony. the fact that all their games have "Achievements" is a plus which helps to make games a little more interesting ...and the fact that some games look and run slightly better on this console can't be overlooked. also, popular titles like Fable, Halo, and Gears of War have made the xbox their home, and can't be bought for the playstation system.
i just said the hell with it and bought both consoles.