Now I don’t use bins, not being blessed with binocular vision. I use a monocular, which has the advantage of half the weight and bulk, but need a special technique to hold steady. However I have borrowed friends bins leading to the following comments.
The arguments about power and aparture are all well and good, but how come nobody mentioned subtleties like eye relief? As a spectacle wearer I find most bins do not let me get close enough to the eyepiece, even if the rubbers are folded back, giving the effect of a small field of view. Take off my specs and I lose the astigmatism correction, resulting in a fuzzy image no better than a kids toy!
One thing I have noticed is that some quite expensive bins give lovely bright images, testimony to the quality of coating of optics, but have distinctly ordinary optical performance. Little things that might not be noticed, like chromatic aberration showing up at the edge of the field of view or a trace of pincushion distortion…but I’m fussy!
Now I know that this will make the Leica or Zeiss fans shudder, but I would go to a camera shop and look on the second-hand shelves for Zenit (Zenith) bins or monoculars. They were made in huge numbers in the old USSR, they were cheap and cheerful 8X30s but they performed well, sneered at rain, and were bomb proof. I have seen a pair go skittering down scree on Tryfan and they were undamaged. This is why I recommend them, they will take punishment, and the mountain environment is very good at handing out punishment. My Zenith monocular has been on countless outings with me and is good for many more outings than I am!
To finish with an anecdote, in the dim and distant past I knew a guy called George Alcock, who had the bins to end all bins. They were ex navy, had 150mm, yes 150mm lenses, and he had them mounted on an old dentists chair. With these mammoths and a phenomenal memory for star patterns he discovered five comets and four novae. No good for SOTA, though!