Simulating a thether holding you back isn't so hard. However, it's a bit more complicated. For examle, you can use a thether to nullify your speed. On the other hand, you can't use it to accelerate (actually you can, but only in the direction of the thether itself). for anything else you have to rely on things like climbing around on the surface of the craft, jumping off with your legs or arms, etc. basically the whole stuff monkeys do in trees. If anyone can simulate that convincingly in Orbiter I might take a plane to his home country, knock at his door and personally take off my hat to him...
Until then, I think MMUs are a more immersive system, because they act at least realisticly in the way they move, allthough we don't have spacesuits with built in gas jets yet.