I have the 13nm Astonomik and 7nm Baader H-alpha filters. At first I tried the 13nm, but ended up with fuzzy stars or haloes, for some reason (and apparently not because of no IR blocking). Also, on nights with a bright moon, the sky background was still noticeable.

Now, I use a 7nm Baader filter. The stars are better, but still slighty fuzzy, compared to shots with a Hutech IDAS or Astronomik UHC filter. Maybe this is normal with H-alpha filters? For some reason, the haloes always seem elongated in the horizontal direction of the DSLR.

The sky background is darker with the 7nm. On his website, I believe Samir Kharusi recommends 6 or 7nm filters, if you have really severe light pollution.

One problem with the Baader filter is that the mounting ring is too wide to fit inside my old Nikon 800mm telephoto lens (rear filter slot). The astronomik and IDAS filters fit OK.