In reply to oceannash909:
What sort of alpine climbing do you do and where? It does rather depend on the kit you have or intend to purchase, your experience and the sorts of routes you want to do. For the European alps if you are using huts or doing shorter 'day hit' routes then a much smaller pack can be used such as a 25-40 litres ( or even smaller ), great for alpine and climbing but then it would not be suitable for multi day hikes. If you regularly bivvy / camp out then a larger pack, perhaps 40-50 litres would be more suitable. 75 litres seems excessive to me unless on an expedition or in a more remote area.
One tip would be to put all the equipment you regularly actually use for the alpine routes you do into one pile and see how many litres your pack needs to be. ( You can borrow some rucksacks or dry bags to see what size suits ). If the pack is designed for it bulky items such as a helmet and ropes can be on the outside of the sack so you can have a smaller lighter pack for actually climbing in.
Another option is to go with a pack that can be adapted. For example in the Crux range, others are available, there is a 50 litre pack that can be used to carry heaver loads but then stripped down by removing the waist belt, lid, back supports to make a much lighter and streamlined pack for climbing in.
Above all else, once you have chosen a size that suits your needs, try different ones out with loads in ( ropes can be useful for this ) as rucksack fitting can be very personal.