Combat Arms is good as a relativly free fps goes. However, the game is usually a hacker haven, and you can't do shit past a certain point without spending some real money, as the better weapons in the game need money.
Counter Strike is more balanced and a wider variety of modes. The aiming system is slightly more complicated, but it's a much more entertaining gameplay experience once you learn it. Plus the zombie mods kick Combat Arm's lame attempt at zombies.