Need a caliber and how far you want to reach out to determine if you 80 will get you there or of you need the extra 20. The extra is for when your scope won't reach the distance you need when maxed out. I have 3 only because they were the same price and I figured a little extra cant hurt. Guess that's my answer if it costs the same or close a little extra never hurts to have.
Just look for a leather sling, I don't imagine leather has magically gotten to be less durable than it was before.
I like single gun cases because I'm scrawny and two get heavy :-). I have a cheap Plano that will hold two rifles, but with the larger scopes it doesn't really work. That is my only experience and I usually have a bolt gun and 870 in it.