Starting at the beginning it’s important to note that this is not going to be a straight volume comparison because the brands simply do not offer liter for liter packs. GG has a 40L and a 50L to Osprey’s 45L.The tail of the tape: The Osprey specsThe Levity 45L in the large model is actually 48L and weighs 1.9 poundsload 5-20lbshipbelt with no pocketsload liftersno shouldter strap pocketscosts $2503 huge stretch compartmentsnon-detachable braincompression static rope with loops all over the place.The Gossamer Gear Silverback 2017 The Silverback 50 comes in at 2.5 poundsload 35lbshipbelt comes in 3 sizes which I assume is the belt length and not the twin pocket size.no load liftersno shoulder pockets2017 model is offered at a discount $1153 huge stretch pocketsdetachable braincompression strapsremovable sit padNot many differences between the 2017 and 2018 Gorilla 40L models except the brain. The 2018 Silverback has not been released if there is one.Just to note; the 2018 Kumo added shoulder pockets which are good for a smartphone, small camera or some snacks but not good for a Smartwater 750ml bottle.The problem with the Osprey Levity 45L/60L is that the load range is 5-20lbs/10-25lbs and so you have all that extra room but there is a tradeoff between volume and weight and in most cases when you start going UL or SUL you also need less volume.For example the John Z. used his Palante Pack as part of an unassisted record out west. Watching his start I remember he complained about pack weight because he had 10 days of food (approx 20lbs). And then there was his gear which must have been between 5 and 10 lbs. Frankly that’s not a lot of gear and given the volume and weight ratio he was at capacity.I have a number of packs and I have yet to find my overnight sweet spot. I think a base weight of 8lbs is good enough for an overnight and probably a long weekend to maybe 5 days. The general difference between an overnight and 5 days is going to be consumables and the effect of weather conditions.