In reply to whenry:
Is this "multipitch climbing" full-on alpine routes or longish valley cragging style rock routes?
For the latter - waterproof, bottle of water, shoes, guidebook - I find the Pod Granite 16 excellent.
For alpine routes - the above plus food, belay jacket, minimal bivvy gear etc. - I like my OMM Villain. It's overkill capacity wise but can be stripped down to very light and frameless, and I find it distributes the weight flatter along my back and therefore climbs better than a smaller pack where the weight sits in one compact lump.
There is a 25 litre version of the Pod which might cover both jobs adequately.