Personally I prefer 7x40 or 10x50 for binos, but that's very much a personal preference. At more than 10x, it gets too tough to hold them steady when looking up. If you're going to get a mount for them, you might as well get a small telescope.
Some of these binoculars used for astronomy are virtually nothing more than two small telescopes joined together. The advantage of using a large pair of binoculars to a similar aperture telescope is that you are using both eyes, which will allow you to see more.