[isoka]

Outdoor, SKs can solo pretty much anything by fear kiting. It will take time, but SK won't take much damage.
Monks are great tanks and should be able to keep agro without too much pain. However, they don't have any snap agro ability such as taunt or casting disease cloud multiple times. But with good haste, damage output is enough to keep agro from any class but rogue (or wiz chain nuking), though rogue should evade and monk should remain tank.

Someone said knights will get 2H upgrade, but monks will also gain triple attack at some point, so monks win hands down when it comes to dps output.

Monks can pull very fast and efficiently (sneak mem blur is golden) and they also have mend which increases their survability.
SKs can invis without caring of charges which give them an edge over monks when it comes to move in a dungeon. They also can split easily by using snare and FD though it takes more time.

In the end, both are enjoyable. If you are more into DPS while being Tank, go for monk. If you are more into versatility while being Tank, go for SK.