Halo goes on kills only, not your kill ratio, so in an every man for themselves game if you get , say, five kills, but die 6 times with the points spread out over the other players, you can still win, while one of the other players may have a better kills/deaths ratio than you (this annoys the hell out of me, as I average a three kills to one death ratio, even online. It's sad but I'm quite proud of it).
In a team game points are added together for the whole team, so people that die loads are more of a liability.
The more players in a team the better, as this allows you to employ tactics and cooperation more- you can have two guys manning a warthog, while another covers with sniping, with a fourth player the snipers back. Depends on how organised you like to get. CTF is my favorite for this reason.
