I get asked this a lot.
"What rope do you use and how long is it?"
These are my general comments and suggestions for alpine and ice use. There is always an exception so nothing is written in stone. I own a number of ropes just so I have options.
l have partners who prefer 70m ropes. But I've never ever seen a place we needed one over 60m. The best they have done to justify the extra weight and mess is use a single 70 to fook up a perfectly spaced set of 50m belay and rap anchors. Seems silly to me. For a guy who climbed several decades on 150' ropes and found 50m ropes LONNNNGGG, imagine what I think of a 70m rope?
Make the climb long, hard and scary enough and I might consider a set of 70m Twins a blessing just for the ability to do a longer rap. It might just be enough mental support to keep me mobile and going up.
Beal Ice Twins
Added useless weight imo. But I do own 3 70m ropes. The same ropes that seldom get used just for that reason. Too heavy to carry or lead with. 70m pitches turn out to be a really heavy rope imo.
I do own a 30m twin sold as a glacier rope that I use in the summer a lot and a 60m twin that I might double and use as a 30m twin as well. I will generally but not always plan on simu climbing a good bit with the 30m rope set ups. The tag line gets used the most as a 30m rap rope when soloing. So I hope the raps are short!
Beal Joker used as a single here and a Beal Ice Twin as a tag line for the rappels
Ropes are a system. Do a 150' free hanging rappel on a lwt single 9mm and a 5mm tag line and you'll figure that out quickly enough. And it isn't all that humorous. Any mismatched ropes aren't all that fun rappelling actually. The smaller rope will run faster in most cases. And a 7mm matched to a 9mm isn't much different. Either way it is smart to check you proposed system first so you aren't surprised. You'll also want to make sure your descending/belay device is up to snuff on skinny ropes. The most recent BD Guide wasn't on two big rappels I did a couple of times recently. The Midi bridge and off the Pencil on Polar Circus. From experience I can tell you the latest Petzl 4 version works a LOT better on the skinny Beal Ice Twin ropes.
actual rope weights?
Beal Joker 7# 8oz
Beal Ice Twin 5# 0oz
5mm tag 2#10oz
As you can see lots of tasty talk on ropes but in the end a set of twins at 10# 0oz is better than a a Joker and a skinny tag at 10# 2oz. Easier to split up and pack the twins, much easier to rap on if required. Easy enough to haul on one if that is needed.
Given a choice I'd rather just use a 50m (or a 60m) lwt single like a Joker if I am brave (I will not fail or have an accident and need to retreat) and know it is a walk off. But if you know you'll be rapping..I want to use lwt twins. Double ropes BTW just tempt me into using one as a lwt single rope. So I simply no longer buy them. YMMV on that rational.
There are lots of good choices in ropes and rope systems. I happen to be using Beal ropes at the moment. But it a very distinct and personal choice, as I buy all my own ropes. They are not given to me. Just make sure you know why you have decided on your rope system.