In reply to jhw: For AD routes like you are planning a 50m single rope is sufficient. I have done the N ridge of the Zinal Rothorn, Hornli on Matterhorn, the south ridge of Dent Blanche and a 50m single was more than enough for all of them. The weight of carrying another rope for abs would cause more of a problem than it would solve for the occasional ab you have to do on each of those routes. On all of those routes there are a few obligatory abseils on descent, but they are all short and there are plenty of anchors to use.
If you are planning on doing long Alpine rock routes, then it's another story and I always use double 60m ropes. That extra 10m of rope gives piece of mind on abseils.