In reply to Ian: I haven't done any long backpacks for a long time - but I go on climbing trips where I camp of a week or a bit more, so have some experience with heavy packs!
For example I was in arctic Sweden at the start of May - we were up for a week and I think on the ski-in my pack probably weighed about 25kg maybe a wee bit more. With my skis on the pack for the bits with no ski-able snow it must have been going toward 30kg.
I have a macpac ascent. I picked this pack because I wanted one that would last well, carry heavy loads but no be a major hassle for day climbs or skis once I got where I was going. The packs weight was a major consideration. Before this I had a Berghaus Cyclops II Alp, which is a great pack for big loads (better than the Macpac) but heavier and more annoying to use as a daypack. My Berghaus is now 12 years old and still usable (it has been used rarely since I got the macpac), and the macpac 7 years old but getting pretty knackerd now.
I would go for a fixed back system as they are both stronger and lighter.
I think expedition climbing makes almost impossible demands on picking a rucsac! You want one that will carry 30kg with out killing you (it is never fun!) but then feels like a 30 ltr daypack when you are doing light weight day routes from your tent. The stripable big sacs from Macpac (Ascent XPD) and Pod (X-Pod?) are perhaps the best compromise, although someone on here said the XPD can get trashed easily.
If you want a pack more for long trekking, sorry but I can't help you as I don't know anything about them. My mate who I was in Sweden with had a Jack Wolfskin which he said had one both US and European tests for big packs. It was huge and had loads of features, but weighed a ton - more than double my macpac I reckon. I was thinking of what that weight difference meant I could carry and still have the same weight of pack: more clothes or food, some more climbing gear etc etc.
One thing I have found that might be of interest if you are mixing a big trek and some day climbs is that those 'spider' compression sacks work fine as a day pack. I did the south face of the aigulle du midi using one as the seconds pack carrying 2xhats, gloves, light waterproofs, and a two litre camelback plus some food for the day. It was great sitting high above you harness and chalk bag and being suprisingly stable.
obviously it won't be so good if you have an ice axe you need to stow - and don't even think about attaching skis to it - I tried but a bit of a disaster!
