For the raid game, every guild will need a decent supply of both. Warriors tank the main bosses. SK (and paladin) do a better job of grabbing and tanking the adds and such.
In the group game with raid gear, it doesn't matter. Both will tank better than is needed.
In the group game with group gear, it depends on everyone else in your group. With people that can't manage their aggro, SK have a larger aresenal of tools to keep the aggro on them. When going up against hard hitting named, the warrior will hold a slight edge in damage mitigation.
In smaller groups, the ultiity of the SK puts them ahead of the warrior.
In solo or molo situations, the SK is way ahead of the warrior.