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My advice above all would be to try it on before you buy it.
Having a pack that fits properly is nearly as important as having a shoe that fits properly.
Also, get a lot of training miles in carrying the pack before you use it in an ultra.
I have a battered old rucksack with bladder that cost me about £10 around 3 years ago. It looks rubbish but is the most comfortable sack I have used. As said you need to try a few and see what works for you....
Without spending a fortune I can recommend the OMM adventurellight 20 and last drop 10. The good and bad bits....
Both are really light material so weigh less than many other brand smaller packs. 10 is a great training runaround, but I've used it for ultra's before. The 20 has more padded straps and two pockets on waist strap for easy access, the angled mesh pockets are great too as you can get two bottles in them and retrieve easily on the move. I'm a convert to the ease of bottles rather than faffing refilling hydration packs and never knowing how much liquid I have left, so the bottle carriage is important to me. The raidlight packs with front bottle holders look good to me, but expensive and from trying on the OMM pack I find bottles attached to front straps bounce too much (maybe just the OMM system?) and its nice to have them out of the way, accessible just above waist height.
The 20 can also be compressed down quite a bit so it fills the capacity gap between 20 and 10L packs well for me. Despite less features I love the 10, compact and sits tight to your back (neither bounce, unlike many ultra runner's packs I see - that would drive me mad over several hours), also has side meshes which bottles can go in, but due to size of pack these aren't angled so not as easy to withdraw bottles while still running.
Never had 'rubbing' problems I can recall and the 20 has seen me around a 100 and several 50+'s. never filled the 20 other than when doing Coast 2 Coast and nearly did on Fellsman where there's a strict kitlist and I've downsized my gear since then.
Though have to say that Saloman pack Squeakz recoomended looks really good. Does the price reflect the innovation? Does the mesh vest bit cause the chest to heat up any more than with a standard vertical strap pack.
I have completed a Ultra with the Raidlight 14lt (the 10 has grown to 14) and would recommend. I also use bottles, its a lot easier to change drink flavours/contents at each aid station than with a bladder. I found I could reach the waist bottle very easily and same with the side pockets. I have tried the 5ltr omo ? and it felt like a childs rucksack on my 6' frame. I have tried the add'l bottle holder on the straps, but found it feeld heavy on the chest when full, ok when half empty but makes a great storage for the event map/Buff...It has the important waist pocket and a nice internal pocket for keys/money etc.
The quality seems to be better than the previous 10lt, which was one reason I orignially went the OMM route.I also have an OMM 15ltr which is great but the Raidlight wins for me due to the add'l bottle storage, however the mesh waist pocket on the OMM is better to store a compass if you need to access it frequently. Its a case of seeing how much kit you need to carry and then finding out how it fits, my orginal intention had been to get a smaller sack not same size
I have also purchased the small raidlight pocket for a waist strap, which holds a good supply of food and gives another option if your chosen sack does not have enough pockets, I wanted to avoid having to go inside the rucksack during the course of the event and in this case I was looking for storage for 8-9 hours of food