The M91s are more collectable especially if you find one matching and in great shape. As for shooters though I'd go with the M39. They're beefier, made better, & less of a PITA to strip for complete cleaning.
The M91s often have screws holding the barrel bands in place, usually on the side instead of inside where you think they'd be. They look almost Bubba untill you realize they were put there by Finn armorers. Plus getting the barrel bands off and then back on without gouging up the wood a little is pretty tough.
The M39 also has a great front sight adjusting option that lest you move it with screws on each side. Loosen one, tighten the other, and the sight moves left/right. IMO they have much smoother actions too, and thats something important when shooting a Mosin.