The first important choice to be made is the size of the objective lenses, and for astronomical use they range between 35mm and 100mm. 35mm is a vast improvement over unaided vision, 50mm transmits twice as much light as 35, 80mm are difficult for almost all people to hand hold, and 100mm are very expensive and heavy. Each increase in size gives a significant increase in light gathering, price, weight, and size. A light weight 50mm binocular can be comfortably worn around the neck, and is a good choice. The 80mm binoculars sold by many astronomy suppliers are a mixed blessing.
Quality control is variable, and some give very defective views.
The mechanical requirements for maintaining alignment in these high power glasses are very strict, and most models are inadequate to the task. The oculars are carried as a pair on a shaft, and there is usually a fair amount of play or wobble at that point. Most 20x models show significant color error, usually only the 11x and 15x models are acceptable, and these should be very critically examined before purchase. A good 80mm binocular is an excellent astronomical instrument, with an ability to reveal dim objects comparable to a low power 4 inch refractor. Magnification is the second choice to be made, and is co-determined by the field of view of the binocular.