Originally Posted by Bandmaster
I have an ED 50 with a 27x being shipped to me. This will be my first scope. With your experience, which would be your choice for a 2nd eyepiece, the 20x fixed or the 13-40x zoom?
Use your scope with the 27x for long enough to know how well it meets your needs. If there is a use for which it consistently lets you down, then consider what you might buy to add the desired functionality (if physically possible and available).
Personally, I don't see much point to using any eyepiece other than the 27x on the 50 mm ED if you are using for looking at birds rather than digiscoping, and you are using it on a tripod. The 20x is somewhat brighter, the 40x somewhat more powerful, but these differences are of little practical significance in my experience. If the 27x doesn't do it for you, you probably need another or different (larger) scope.