There are three types of scope - "straight- through" (the eyepiece is one the same axis as the body of the scope), "angled" (the eyepiece is at an angle of abot 45 degrees to the body of the scope), and, rarely, a more complex, shoulder- mounted design. Straight- through scopes are easier to use from, say, inside a car, angled scopes are more comfortable for tall people and more easily shared by people of differnt heights.
The choice of the eyepiece lens determines the overall magnification. The best magnification is decided by a number of factors. Magnifications of less than 20x are unusual, as (much cheaper and lighter) binoculars can provide this. Magnifications of more than 60x lead to poorer brightness is poorer and, even on a tripod, shake can be a problem. The field of view is also limited.
Usually for birding, 20x or 30x is favoured. This gives a good field of view and a bright image.
Spotting scope manufacturers fall into several groups. At the top end of the market, it is generally accepted that three manufacturers compete strongly for the ultimate in quality: Swarovski are the most expensive of all, closely followed by Leica, and Zeiss. Several manufacturers produce scopes that are generally thought to be of almost equal quality to the top three, but at about half the price, among them Kowa and Nikon.