Unfortunately, the discussion isn't so easy.
The F5 surface is harder to make as a F10 surface so you are probably going to be getting a poorer surface with the shorter scope and this means that you aren't going to be having as good in image.
Next is that anytime you put a piece of optic into the light path, you lose light and add in abberations to the light. Thus, adding in a barlow will decrease the image quality.
As a result, I'd go with the F10 for a planetary scope or even something longer so that I can gain more power for the planets.

• Or split the difference - danieldgj - November 14, 2002 - 01:59 UTC