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It all comes down to how much brightness you can stand, eg. your mirror has an area factor of 12x12=144. The filters will give you reduced brightness equivalent to the following telescope sizes for a given magnification:
50% = 144x0.5=72. SQRT(72)=8.5. A 50% filter gives a view in brightness similar to a 8.5" telescope for a given magnification.
25% = 144x0.25=36. SQRT(36)=6. A 25% filter gives a view in brightness similar to a 6" telescope for a given magnification.
13% = 144x0.13=19. SQRT(72)=4.3. A 13% filter gives a view in brightness similar to a 4.3" telescope for a given magnification.
A polarising filter is available but you will notice a significant loss in resolution. I have used one and I can see small craters on the Moon disappear when using the polarising filter with a C9.25. Planetary detail too suffers greatly.
You may find yourself with all three filters depending on the objects you observe and the magnification you select.
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