The same applies for waterproof trousers as for waterproof jackets, i.e. the lighter and simpler the better, as you’ll probably (hopefully!) spend most of your time carrying them. Expensive Gore-Tex is not necessary and a basic model from any outdoors shop will do perfectly well, especially as breathability is less important for the legs. Try to get full length side zips however so you can put them on without taking off your boots and crampons.