What they said

SK can tank the majority of encounters, but they have to do a lot more work for aggro at the higher tiers than warriors do. They also really can't tank very well against bosses that silence/dispell, which a good number of bosses do.
The DPS boost to groups from Death March is AMAZING though, so even if you're not the main tank you're adding a LOT of DPS if you have a few melee in your group. I rarely ever cast the lifetap spell because I was usually spending every waking moment casting spells/clickies to try and hold aggro.
As far as donors are concerned, warrior weapon is more important than SK weapon. At lower tiers the SK weapon is way OP, not quite as much once you get T7+ (though still good). I never bought the BP, but from what I heard while I still played, it makes you pretty much unkillable while it's up. Warrior BP is pretty good for snap-AE aggro, but not totally required. The DI shield is almost essential for all tanks, but it's easier to cast mid-combat for SK than it is for Warrior.
In the end warriors mitigate better and are far better at aggro, two things which put it ahead. SK's are more utilitarian and are slightly better at aggro in earlier tiers, if that's your game. If you go into a fight knowing the mechanics, an SK can tank almost anything. A warrior can tank everything
