There's tons of posts on this very subject if you scroll a couple pages. With that said if you're looking to purchase just one pair then I'd say get 10's. I run 10's on my chest and 15's in my pack. Kind of wish I had 8's but it's what I have. You can cover a lot of country with 10's on a tripod and hand hold them while still hunting in timber. This is my opinion of course.
Agree with above. Only thing I would add is that you won't be able to use 12x50'a without a tripod for long periods of time, so if you're only buying one pair keep that in mind. What the 10x42 lack in magnification may be made up for in versatility.
Any good binocular becomes a great binocular when tripod mounted. The increase in glassing quality is tremendously improved when you tripod mount any binocular