Xbox 360
Pros:
-Greater percentage of users have Mics.
-Graphics will look the same or better unless you have an up-to-par computer that can run the Very High setting on PC
-Possibility of having a giant community.
Cons:
-Matchmaking can take forever depending on your NAT type as opposed to dedicated servers with a small loading time to connect to.
-Can potentially be out done graphically by PC's
-Chances of getting unsportsman-like players on your team are higher
PC
Pros:
-If you can run the Very High setting on a high resolution it will look noticeably better.
-Nice, More tight knit community.
-Keyboard/Mouse controls may provide an easier playing experience as Mouse out does anologue sticks for aiming any day.
-Dedicated servers, You don't have to go through matchmaking.
Cons:
-Possibility of having a smaller player community then XBL so you're more likely to play against the same people
-If your computer isn't up to par you may experience lag trying to run higher settings you also may lag if other players are from other regions (Assuming PC doesn't have Region Restriction)
-Also if you're computer isn't up to par, It can look worse then xbox on lower settings.
Details and Personal opinions:
I just want to go over a few details on some points that I view could be broken down in more description to give a better idea of what it is.
First of all, The Up-To-Par PC issues. If you haven't bought a new PC or upgraded your GFX card on your PC in the past 2 years I can almost guarantee you won't be able to run Brink on Very High settings. So right there, One of the Pro's for PC is gone. On that note, About Lag. If there is no Region Restriction through Steam ( Haven't used it much so i don't know ) you can potentially be connecting to people from Europe and UK, Or North Americans if you are from those regions. If that's the case, Chances of you having a completely smooth game I'd say are unlikely, But possible. That's a risk you're going to have to decide you want to take.
Now onto the community issues. The PC has a smaller close knit community then XBL does. That being said, It's not SMALL small, It's just smallER then XBL. That's a good thing considering most of that community is made up of nice, Team players. Altho, If this game goes BOOM like CoD or Halo did, The amount of players online for XBL could be massive in comparison to PC. Those are both Pro's of each. The cons however, Can be pretty brutal. On XBL more players have mics which is another pro for a team based game, But, there are always, Immature, Selfish, Egotistical players that will do anything in their power to avoid helping the team and/or trying to ruin everyone else fun by being ignorant. But if you have a few buddies you're playing with it's really not that bad. On PC with a smaller community divided among regions certain times of the day can reach a low point for online play. Early mornings and late evening play for the North America will have considerably less people on then during say, 3-7pm. Any times later or earlier then those most players have school, Work, Ect. and won't be playing. Which in turn could mean playing with/against more bots then desirable or playing with players from other regions which could then lead to lag issues. Again, Really up to you which risk your willing to take.
As for my opinion, My PC is made for gaming and can run it on very high settings, But, I'm still going with XBox. To me the Pro's are more in my direction, I don't really care that the PC might have graphics that are a little more polished and dedicated servers. I think having a potentially bigger community will out-weight the cons. I like playing with new people, Keeps things fresh.
I'll also be adding more Pro's and Con's to the list so feel free to add any others you feel should be mentioned.