There are different types of binoculars based on quality, size and budget. While buying binoculars you will find some numbers like 8x42, 10x40, 10x50 etc. Here the first number say 8 or 10 is the magnification ie. how many times (8 or 10 times etc) the binocular magnifies. Greater the number better is the size of the distant bird through your bino. The second number is the size of the objective diameter. Higher the objective diameter, better is the light gathering capacity. So between a binocular 8x25 and 8x42, the 8x42 will be brighter. So between 8x42 and 10x50, the 10x50 will give you a bigger and brighter image. However, generally the 10x50 binoculars are bigger in size.

Then comes the quality of lenses in the binocular. That adds to the size and cost. There are also binoculars with image stabilization technology built-in.

If you are going on a trek, then carrying a bigger binocular can be tough if you have lot of other equipment as well. However, if you are on a safari vehicle or say in a boat in Sundarbans then it is easy to carry. If you are going to park yourself in a hide or watch tower, then definitely get the biggest and best. So frankly speaking without knowing your needs it is difficult to suggest.