PS3 autumn line-up has like, imploded, only GT5 left basically, and that's a maybe anyway. However. 2011 PS3 line-up, damn crap-ton of games; Xbox 360, atm all we have is Gears, and Codename:Kingdoms and the various kinect exclusives (Which tbh, have dubious value to me personally, due to the typical on-rails nature that Kinect forces).
Off the top of my head, PS3 exclusives for next year are: Killzone 3, Resistance 3, Sorcery, ICO/SOTC HD pack, The Last Guardian, Infamous 2, Twisted Metal, LBP2, FFXIII versus, Socom 4, Motorstorm apocalypse - there's some more, but they don't seem to have really caught my attention. FFXIII versus is only listed because I did love FF VI, FF VII was alright, so this will basically be my first FF game since..ages ago, lol.
HOWEVER, do remember that the PS3's online playerbase is..well, not as large as Xbox's. Now, I'm not saying MP is needed for fun, but it does add a heck of a lot of longetivity. Xbox 360's strong suit is it's online as it stands, not its exclusive's - the exclusives it does have are packed online, you will practically never struggle to get a game. Also uh, PS3 pad is not the most ergonomic - there will be a learning curve, haha.
Now, I dunno how much you'd use your console, but, as a PC gamer primarily, PS3 suits the way I play a lot better - If I don't use the console online for a while, I'm not wasting money, and that's nice, I mean, sure, there's a trade-off in terms of experience, but I'm a PC gamer goddamnit. Probably a good point to make is that, I have a Wii and a PC. I'm going to buy a HD console, and am trying to do this logically. Last-gen the Gamecube was my choice, because, heck, I love Nintendo exclusives
(Super-psyched for teh 3DS - KID ICARUS !!! )
*ahem*
Anyway, my point is the main factors to consider are:
How much you will play? Live is a paid service, how much do you use it now? Is it worth it if you play sporadically?
How much do PS3 exclusives um..excite you? If you sell the xbox and then start pining for Halo, that wont be much good. (BTW, how good is reach? I swear, I want/wanted an xbox just for reach LOL)
Is there a capacity to own both consoles? Even if the PS3 is second hand, this is by far the best option. PS3 is quite a single-player geared machine still atm (meaning it hasnt got a super addictive FPS yet), so owning an Xbox for multiplayer is a really solid idea.
I'll probably get a PS3 to supplement my addiction to PC gaming - and then roll on Spring 2011 for teh 3DS
EDIT: ORRRRR
Sell your xbox and save it for the steam xmas sale..a literal mountain of excellent games for cheap! Probably not what you wanted to hear but the options always there!