Even though I hate it, Counter-Strike is absolutely superior to Combat Arms. Combat Arms is buggy and has rather poor graphics, even if you try to compare it to CS 1.6. Source is the best one over Combat Arms, CS 1.6, and CS Zero.
I just recently tried both, and I have to say I liked Counter-Strike much more than Combat Arms. Sure, it costs a little bit of money to buy CS, but you don't have to deal with people paying money to get a gameplay advantage over you like in Combat Arms.
Now Combat Arms has more graphics, any way Counter Strike is fun, but Combat Arms is a free game that gives you a little above moderate gameplay, but Counter Strike is slowly breaking down, my opinion really. Theres quite a few FPS games out there, now they are becoming free, paid games (FPS PC GAMES) are less likely bought. I think Combat Arms For the Graphics now and Counter Strike for skills