I'd say 360 every time, unless you want Blu-ray or Uncharted/Little Big Planet. Although you have to pay for the full Xbox Live service, it's a lot better than PSN – better interface and, most importantly, less issues with lag. You get what you pay for, and I think Sony have realised this as they recently mentioned that they're considering bringing in the subscription model for PSN.
On multi-platform titles, the 360 versions tend to run smoother (less issues with framerate, texture tearing, jaggies, pop-up)and also tend to have shorter load times (unless you do the full installing on the harddrives, then they're pretty similar). Unless it's a PS3 exclusive, the vast majority of devs still use the 360 as their default development platform. And there's some devs that tend to avoid the PS3 like the plague (like Valve, couldn't be doing without me Left 4 Dead!).
The gap in performance is decreasing though, as devs get to grips with the PS3. And, truth be told, a lot of people wouldn't actually know the PS3 versions are inferior unless they had a 360 version running alongside at the same time.
Best to play it safe and get both. It's the only way to be sure!