Mark, as you can infer from many of the posts, you, like most of us want to use our spotter in multiple ways so one of the major things you have to ask yourself is what will be my main application. If you are an established backpacker, or you are a guy that will stick with what you do and will be mostly packing the scope up, then you should go with a 50mm-65mm scope. If you think you will end up doing a lot of glassing from camp or just off the road then a bigger scope will be better. There is a nice benefit that comes will larger scopes, they will perform better at higher magnifications. If you take two of the same scopes, one a 65mm and one a 85mm, the 85mm will be brighter. The only way to have a smaller scope perform comparable to a larger scope is to increase the quality of the scope. Many people will compare the 85mm Vortex Razor with the 65mm Swaro. If you were to make that comparison and found them to be equal you could buy the Vortex to save money, however, it is not a very packable scope due to size and weight. If you can afford the swaro 65mm it will perform great at high magnification (not as good as the Swaro 80mm) it is a 20-60x, and is smaller and lighter then the Vortex so it is still packable. Once you have your price range you apply that the the scopes in question and the way you will use it. You might want to get a higher quality 65mm than a 80-85mm larger scope, same price range.
I have the Nikon ED 50mm and I love it, because it does what I need it to do, however, if I hunt a different area I will be looking to get the 65mm Swaro so I have more magnification and better light gathering. The 50mm's quality diminishes at the high end of 30x similar to the 65mm diminishing around the 40x, so you will get at least 10x the magnification will equal performance. Usually you have a better eye piece with the bigger scope.
