I have both packs mentioned and they are excellent. You can fit winter kit into the Mutant 28 just nicely and it fits into the Variant 37 with room to spare. My impression of Osprey's sizes is that thier method of measuring size is very honest/conservative so that the number appears to be 5-10 liters more generous than that of other manufacturers. The mutant 28 feels like a 35 and the variant 37 like a 45.
Osprey variant 37 is huge, I had the variant 28 which I liked could get a full winter rack and rope in it no bother hard to find now, mutant 28 is a cracking pack, theres new model mutants on the way July 28
& 38,might sell my mutant 28 if ur interested £40
I used to think Osprey were too 'strappy' but I got a Variant 28, no longer made, and love it. It's not a no-frills ultralight pack (of which I have quite a few) but any little extras are useful things, not decoration.
Agree re: sizes. My V28 size Large always looked bigger than 28, and fully loaded in the Alps last year next to my mate's V37 you could hardly tell them apart.
I'll be buying a Mutant 38 when I find a green Large at a good price.