It depends on what you're setting up with.
I went with the PS3 because I had just gotten an HDTV and I really wanted to open it up (and not just with games) so the BR player was a very welcome function and there is a difference, if you have an hdtv. I also have a psp and the two work together really well. Free online was welcome but not a big deal to me since most my online gaming was done on the pc but I thought it was great when I first got it, plugged it in, connected to the wifi and immediately started downloading free demos. I didn't even buy a game for a week, just kept downloading demos for my ps3 and psp.
Another pleasant surprise I didn't realize was very cool: how cheap and easy it is to replace the HDD on (more recent) ps3 models.
It didn't come with an hdmi cable but I just got a $7 one of those off newegg. No headset but that came with Warhawk, which you should really get if you get a ps3.
If you have a good gaming PC, you can play most of the 360 games on your pc anyway: you shouldn't miss out on Mass Effect.
But I have to tell you the thing that really turned me away from the 360 was the rrod problem and the sense that their best long term solution is to just keep replacing the ones that break. I don't know about you but that would enrage me if I was in the middle of an engrossing game. And even that didn't completely turn me off until it was rumored that they rushed it onto shelves and ignored the rrod on purpose to be first out of the gate.
That said, I have a few friends who own 360s (only one of whom has actually suffered the wrath of RROD.... Twice though) and we have lots of fun playing on them.
So I generally lean towards ps3 but if you have an sdtv, don't have a psp, and prefer to play (online) on your pc, then you may as well get a cheap 360 arcade. It's half the price; if you don't buy online service. Then again, if you're really into DLC, the 360 may be the way to go as they seem to get much more of that but I wouldn't recommend the arcade model in that case.
Just be glad you asked on this site (which has very few rabid "fingers in ears" fanboys who wont admit to "their" rival machine's benefits) as opposed to some of the other sites. Of course we still have a few as you can see.