I like both but I like Combat Arms better because of all the different modes and the wide variety of guns. AVA only has Demolition,Escort, anhilation and prisonbreak.
Combat Arms has Elimination/ ELim Pro, 2 fireteam missions, Quarantine Regen, Last Man Standing, ONe Man Army. AVA doesnt even have a free for all mode.
THe weapon system (in my opinion) is better in CA because unlike AVA u can buy and try weapons instead of sticking with 1 weapon for ages.
Well AVA is worth downloading because of great graphics, good gameplay and permanent weapons (although it can get boring and u will find urself making multiple accounts because u wanna try new weapons)