In team modes like Capture the Flag or Domination where team coordination is important to effective play, I prefer smaller numbers of players, since you can actually coordinate something. In other game modes (including non-team ones), I almost always prefer larger numbers of players.
Between duels and 2vs2 CTF games, I guess I prefer duels, barely. I'm not sure what Skele means by "important" in this context. If it's for determining general player skill, then I'd say duels are - in my experience you can be fairly certain someone who's good at non-team gametypes will be decent in team gametypes, but the reverse is less true.