I have already made a post about it in a different thread, I have owned a ps3 for a few years, so I'm not talking out of my ass...
If you want I can maturely discuss the differences so we don't have people flame console war like teenagers.
I've had both a PS3 and an xbox since their release. I've had 3 xboxes break down on me. And I take great care of my systems, this has been my first system that has broken down. I'm on my second PS3. I love Xbox for shooters, Halo being one of my favorite. I never noticed any difference online compared to my PS3, however. I RARELY get timed out on either, and to be honest it happens slightly more on xbox. My internet connection is beast, it's wired and the fastest Time Warner has to offer. To be honest I think XBL > PSN or vice verse is totally preference. If you like the interface on XBL you'll gravitate more toward XBL, naturally.
As for MK, the extra character, extra stage, 3D compatibility, MK Movies on Blu-ray, better d-pad, takes it for me. I seriously doubt there's gonna be a tremendous difference when online play comes into the equation.