In reply to Tom Ripley Mountain Guide:
I use a Joker for easy big mountain routes in the uk where I'm going to be doing all the leading, (Tower Ridge with a Douglas Boulder route to start, North Wall Route on Buichaille Etive More, The Great Ridge with Direct Start on Garhb Beinn leap to mind,) where others may concider just taking a half rope. I like the security of a thicker rope and even on quite a big day don't think the weight gain actually slowed us down. If I'm in a position to do some Swiss big easy slabs in the Summer I'll use this set up. It must be said that I use this rope primarily to save faffing with two ropes on change overs rather than any scientificy / fall factory / clever reason!
However if I was going to the Alps to do some big routes as a three I'd probably take, (because I've already got them,) two 9mm - a bit more reasurring than really skinny ropes.
My experience is that using a single skinny rope is great right up to the point when it really matters - when you're looking at a fall onto a rope over an edge etc. My advice, for what it's worth, would be to forget about all the supper light Alpine stuff we read about - imagine yourself in that sort of position and then ask what the thinnest rope you would be relatively happy on would be. Then take that rope.
You need to know that I write this as a Dad to two young lads so my opinion on what to use in these situations has changed a little from where it was when I was in your position, however really ask yourself if saving on rope weight, (as long as you are still careful with other weight saving areas,) is really going to make a huge difference to speed.