Note reducing the aperture also reduces resolution. Not so much an issue on the Moon although smaller craters will no longer be seen. On planets fine detail will also be lost.
Stay with full aperture on the planets and during moments of exceptional seeing you will see much more detail. On the Moon a good quality neutral density filter is best and these are available in 13%, 25% and 50% transmision depending on your observing preference. I would not recommend variable polarizing filters as some (if not all) can also cut your resolution significantly. I have noticed small craters on the Moon actually disappear when using a variable polarizing filter to no filter, this on a C9.25. Not sure if there are better polarizing filters out there but a quality single density filter would not have such resolution issues.
See
http://www.lumicon.com/telescope-acc...ensity+Filters for more details.