it depends what games you want to play. look at the games YOU want to play then from that decide what machine. i would personally say that the PS3 will be the better of the two, well four if you include the shitbox and wii.
at the moment i own an iMac (top spec), PS3 and a new MBP and to be honest the only games i play on the macs are WoW. i just cant be bothered to boot into windows - the only thing i will boot into windows for (and thats when its released) is Half Life 2: Episode 3 and GTLegends. I mean even Starcraft II is going to come out on Mac OS X.
anyways back to the point - the ps3 is a lot easier to use and you dont need to mess around with drivers, viruses or anything like that. and all the peripherals work on the ps3 flawlessly. ive got a logitech g25 steering wheel plugged in (for gt5 of course) and its excellent. then a Saitek X52 Pro works with no problem.
in terms of online capabilities, well i will say now that the computers are a lot easier for connecting to other servers, but in no way more advanced. for example resistance 2 has 64 (or 60 w/e) man servers, and then GT5 has 16 player races.
you also have the added bonus that its a great media center and you can, if you want plug in either an epson or canon (i think) printer. as well as any USB wired/wireless mouse and keyboard and you also have the web browser with flash support (so youtube for example).