At Aiguille we will often first ask you what is the main end use for the rucksack. Try to be brutally honest with yourself at this point. If, for example, you use your sack for summer cragging and Scottish winter buy a sack for that end use. The fact that you may be hoping to spend a couple of weeks in the Alps in two years’ time should be the time to compromise with your sack, not the other way around. By buying a sack principally for the Alpine trip (which may never happen) you compromise the main use of summer cragging and Scottish winter.
On the face of it the requirements are very similar, carrying ropes, rack, helmet, harness, etc. and maybe ice axes and crampons. The big difference is the British weather. Scottish winter conditions rely on bad weather and it follows that we are often climbing in quite poor weather. On the walk in the sack is full, but once the rack, ropes harnesses are in use the sack may only be a third full if that. When spindrift comes pouring down a gully or crag you seal your neck or put your hood up thereby reducing the amount of snow that gets in around and down your neck. On a fixed lid rucksack it has in effect got its hood up and the vast majority of snow will stay out of the sack. However, on an extendable lid sack the snow can get down the back of the lid into the sack. Not ideal at all.
This doesn’t happen so often in the Alps because generally we would not go climbing in bad weather. An extendable lid in the Alps does in fact have some distinct advantages. You are able to overload the sack which, with the bivi gear and food (not needed on single day excursions in Scotland) is a vital necessity. Once at the route and the climbing paraphernalia is unloaded you can drop your extended lid inside the rucksack and do up the draw cord, stopping the heavy lid pocket hopping around as you climb. So start by deciding if you really need an extendable lid or not.
A climbing sack needs to be ‘bombproof’. Meaning it needs to be strongly built from materials which can withstand the abuse and rigor of both the mountains and travelling. It wants to be lightweight as the less weight we carry the faster and easier the climbing can be. The Holy Grail, therefore, is a bombproof sack that weighs nothing at all. The reality is a sack that is very robust and durable and weighs in at 1-1.5kg. This is for a 45-50ltr sack. It would be easy to get this weight down to sub 500g but the sack would then be much less durable.
There are easier ways to save weight. Try this out, empty your mates sack that they reckon the contents of which they have pared down to a minimum. Weigh what you think they can leave out. Let them do the same to yours. The resulting ‘discussion’ should keep you entertained for years. Get the point? We can always reduce weight so why compromise the durability of the item which has to carry it all? If you rip the side out of a lightweight sack on a haul or chimney high on a route, life can get pretty interesting if not downright dangerous if all your kit is spread out several hundred metres below on the glacier. On balance 1-1.5kg isn’t that much is it!?
So to keep the weight down get rid of all of the unnecessary features and keep it simple.
UK cragging and winter, we are really only talking about day routes for 99% of people, so the load should never be ridiculous. If it is then reduce it! We could, therefore, have a sack with a fixed lid and a fairly simple padded back. It then just comes down to capacity. Try looking at the Cirrus, Stratus, Bora or Zenith with capacities between. 33ltrs and 47ltrs. We are all different in the amount of kit we want to carry (weight again!) but remember that a sack that is not stuffed out solid is easier to use and carries much better. Precision packing a sack that is only just big enough on top of the Ben in a full blown blizzard will not win you any favours with your mate/s who are waiting for you!
For the Alps we could well be talking longer walk-ins with more kit (bivi, food, etc.). Ideal for this would be a fully featured twin back stay 65ltr load carrier. The snag is that, once on the route, a sack like this would be an nightmare. So we compromise and try and build a sack that can carry all the kit (extendable lid) in reasonable comfort (some back support) but once ¾ empty does not overly hinder us on the route by having too many straps, too much padding and too little head clearance. Try looking at the Zephyr or the Stratos/Bora with modifications.
If your Alpine climbing is lift fed day routes or from huts, your loads will be less and so your requirements are more like cragging in the UK so the earlier comments apply.
In the end, we are all individuals with our own way of doing things and what suits one may not suit another.