Hey Terry,
Good question:
Lengths:
I often use a 50 meter (gasp!) rope for alpine stuff when the approach is long. Vince and I used a 8.5mm half(50m) and a 5mm tag(55m) on the Rupal Face. Sure it was more rappels, but the raps were not the crux, as much as carrying all that shit up there. Marko and I used 50’s on the North Face of North Twin for a similar reason. It is something to consider when the approaches are long. Though anymore I have to get a 60 and chop it to 50, which always feels a bit odd.
Remember that if you pair a dynamic rope with a non-dynamic (no rope is truly static) tag line your tag should be at least 5 meters (max 7m) longer to account for the stretch under body weight of the dynamic rope during the rappel.
Also, I’ve found that, especially when you’re climbing older, classic routes, the pitches tended to be shorter then (than now). In the Dolomites for example, 15m and 20m pitches are really common, but a lot of those routes were done pre-world wars or in the 40’s and 50’s. So they had short ropes and small racks (and were damn good climbers!).
I never use 70 m alpine climbing due to weight but I do use them/like them for straight ice climbing.
Keep in mind that the 8.5mm Opera is 48g/m and that puts a 60m at about 2.9kg. My 65meter 5mm tag line is about 1.5kg. 4.4kg total for a 60 m set up.
If you go double ropes and drop to a Beal Gully 7.3mm rope the weight is 36g/m. Which puts each rope at about 2.5kg, so 5kg total for 2 x 70m ropes. 4.3 kg for 2x 60m ropes. Within 100 grams of the single plus a tag set up.
The real burner for me on single vs double ropes is the rope-work involved while climbing. A single is WAY easier to manage while climbing and leading. And that saves time. And I’m good a placing runners so I don’t get rope drag. And I am good at rapping on the single+tag set up so that doesn’t slow me down.
Bonus pro-tip: Carry one DMM Revolver (wiregate) biner on your rack for that one sharp turn on the rope’s path, completely eliminates most rope drag.
So this is a long way of saying that I never use half/double ropes anymore. I use some length; 50, 60, or 70m Beal Opera 8.5mm single rated unicore rope and a 5mm tag line.
For sport climbing I go up to the 80m (lots of modern sport routes are longer now) Joker 9.1 just so my belayers aren’t freaked out, but if it was only up to me, I’d use the Opera all the time.
Also worth noting, whenever I climb with 3, I use 2 singles. I didn’t use to do this, I used to do alpine routes in the Rockies on 2 twin ropes with 3 climbers. Makes me shudder now. The chance of a half rope getting cut in a swinging fall while seconding is, to me, much higher than with a single. Again, I just don’t use halfs or twins anymore.
cheers, Steve