Go with the f5.6 mirror. There are several reasons. First, unless you want to spend a small fortune on eyepieces, the f5.6 will work well with the commonly available (and reasonably priced) Plossel types. Unless you opt to spend lots on some of the Nagler or other pricey wide field types, there is no advantage to the f4. Second, you can use a smaller diagonal with the f5.6 so the obstruction loss will be smaller. Third, the f5.6 will collect as much light as the f4 and the contrast will be slightly higher (the sky background will be darker).

There are more reasons but these are enough to help you with your decision.

Jarvis Krumbein