In reply to robatkinson:
think what you want in more detail than just days and bivvys.
it can be safely said that 55L of MAIN PACK space (ie dont include pockets) will cover most scenarios up to about 3 days (after day 2 its really only food that changes).
with so many packs now weighing about 1000gms when stripped, its not hard to get something big that will fit all your stuff INSIDE. even a rope.
this leaves function as your main priority, and being able to chuck everything in fast and not have to repack a jigsaw everytime a function to put high on the list.
wish i had a dollar for everytime someone has wowed me with some fancy daisy loop or gimmicky tool attachment due to its 'function', but then not fitted everything in because the whole pack was too small.
for climbing you wont need a real waist belt. also avoid a lid.
any tool attachments gizmos you cant use with gloves will be a problem.
straps for cinching and in case you need to attach stuff need to be looong.
a back panel is important for carting pointy gear.
after carrying lots of packs for all sorts of reasons i settled on one of MHWs latest (i use the south col, but the diretissima may suit you better).
strippable to 1000gms whilst still retaining real carrying function, its big enough to cram everything into and solid enough to still haul 20kgs properly.
every buckle is glove friendly, the tool attachment things are too, and theres a big dyneema pocket for sharp stuff and to protect against abrasion.
for cinching and securing theres all sorts of strap configurations, even able to remove or tuck away.
take it from someone who guides all winter and has collectively spent about a day waiting while people try and jam too much into a pack, then fiddle about strapping stuff all over the outside, only to be convinced that the 200gms saved on a pack too small was somehow worth it...