In reply to roamer:
I've used the Guide 35+ for just about everything over the past 3 years, from summer scrambling to a 4 day winter hillwalking camping trip. (The back system - if it's the right shape for you - is very supportive and comfortable, even with a big load. Well worth bending the metal bits to suit your back shape.) Definitely roomy enough for winter climbing: the extra litres afforded by the floating hood come in handy when gear is stuffed back in in a spindrift fest at the top of a climb without much attention being paid to neat packing! And the zip access to the main body of the rucksack is very handy for getting out stuff that's ended up at the bottom. Useful pocket under the hood. Mine has been used a lot and shows virtually no sign of wear.
Also handy to have the ski attachments if you tour. These can be adapted to carry other things too. And for mountaineering days there's a gear loop on either side of the waist band - surprisingly useful!
The only down side of the durable construction and supportive back system is that it weighs more than a number of packs out there.
As others have said though, the key thing is to try on as many fully loaded bags as possible. Ropes make good ballast for this and ought to be in plentiful supply at any retailer!