There different in the way you play. Halo tends to be fairly fast paced and the player is usually constantly moving. While teamwork is valuable, being alone doesn't mean complete destruction.
CS is usually at a slower pace (especially since you die, your out) and relies mostly on teamwork exclusively, as you cannot take on several people at once with a high rate of success.
They are each great games in their own right, but it's hard to compare them as similar titles.