I used a 65l Osprey sack for my first two challenges.

There were two problems. i) The metal frame seemed to be too wide for my body (size is usually chosen based on back length)

and ii) more significantly the waist band was too thin and insubstantial to support the 14/15kg or so I carried without bruising my hips.

The model I think was an older version of the EXOS.

Possibly the packs you are considering are heavier and have better support.

Weight in my opinion can be the biggest factor in having a wonderful TGO crossing. Osprey packs are superb but I prefer a Duneemo pack from HMG (windrider) or Zpacks (ArcBlast) just to drop off almost a kilogram of weight. 55L capacity is more than adequate. WEIGH EVERYTHING then eliminate any grams you can. Some people have over a kg of toiletries when that can be easily below 300 gyms. I have used Osprey Talon in the past. As noted by others make sure to get fitted properly to have comfort and eliminate hip of meralgia issues and such. Base weight can defo be below 10kgs and below 5 if you work at it. See you at TGO. I start from Mallaig año my fourth go.

A windrider 3400 is a good pack for sure. Good luck selling it.
Dara

Used a few Ospreys back in the day, as others have said they are good packs but be sure to get one that fits you properly. The app should help otherwise actually trying them on and getting it fitted properly is super important.

I'll be bringing my ULA pack for my third Challenge. ULA hasn't let me down yet and you can't get new Golites anymore (although i think they are up and running under a different name with the same patents.)