The first thing to decide is how you will most often be using the rope: Do you need a dry rope? Will you be mostly top-roping? Will you be doing multi-pitch routes or long approaches in the mountains? Each of these factors affects your decision as to diameter, length and dry/or not of your rope. So lets look at your choices:
1) Single ropes – 9.4mm-11mm. The larger diameter, generally, the longer the rope will last. The larger diameter ropes are harder to feed through belay devices and are heavier. Conversely, the smaller diameter ropes are lighter, but do not last as long. I would choose a 10.2mm for top-roping and on big walls, where I have to do a lot of jumaring. I would use a 9.4mm for alpine routes, and a 9.9mm for most of my climbing (I used to use a 9.7mm but it was discontinued. It got so fuzzy anyway, that it felt bigger than my 9.9mm)
2) Half ropes – 8mm-9mm. These are two ropes used together in such a way that you clip one line of pro with one rope and another line of pro with the other to reduce rope drag, or simply alternate clips. If you clip both strands into the same piece of pro, the impact force goes up on the pro and on you (not good). The advantage of this system is that it reduces rope drag on wandering routes and you have two ropes in case one gets chopped. Also, you have two ropes to rappel. The disadvantage is that it takes extra time and is more awkward to manage the ropes while belaying on a hanging or semi-hanging belay.
3) Twin ropes – 7mm-9mm. (PMI makes half and twin ropes in 8.1mm and 8.6mm). Another two-rope system, but here you have to clip both ropes in each piece of gear so there will be more rope drag than with half ropes. Like half-ropes, you have two ropes to rappel but have the disadvantage of dealing with the extra time and awkwardness of managing the ropes while belaying on hanging or semi-hanging belays.
4) Other factors – the most common lengths are 60m and 70m. You can normally get away with a 60m rope (and this saves weight and money) except on some long single pitch routes. As for dry treated ropes – the water-resistant coatings are often applied to the sheath and to the core fibers as well. This makes the rope more water-resistant, stonger, and it lasts longer.
So what to make of this? I have run out of space, but you might want to find out about fall rating and impact force and how to care for your rope. Ropes do have a shelf life and at most, a rope is only good for 4-5 years. I use PMI ropes (don’t have space for testimonials here). I get a 70m 9.9mm dry rope at the beginning of every ice season and try to make it last a year. I also carry a 70m 7mm dry cord (static) with me on multi-pitch routes for rappelling.
– Kitty Calhoun