Which one COD 8 or Battlefield 3

It all depends on what game you prefer. If you like an very realistic type shooting game, got for Battlefield 3. However, most games on Battlefield are all about objectives.
Also, Battlefield 3 isnt a quick game. If you have only something like 15-20 minutes free time, Battlefield isnt the game to go on. A match will last a long time if both teams are very good.

Call of duty - Well, if you want a 10 or less minute game, call of duty is the game. Call of duty isnt as realistic as Battlefield but COD is the most popular (for now).

Its a personal option and only you should pick on.

For me, I'll be getting both.