Wow. There are some "interesting" arguments in here.

Whether you go with an 11x6 or 11x5 makes little difference, and should be based on how you want your airplane to perform. Buy each, and find out which you prefer. In general, the 11x5 will give you better acceleration, which when training can be important at times. The 11x6 should give you just a hair more speed, but not necessarily enough to be noticeable.

Try them both, and see what you like. Stay away from an 11x7, as that is over-propping the engine in my experience.

Also note that different brands of props will perform slightly differently. Try some different brands as well, and see what you like.