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Hydrogen Alpha emission occurs at a very specific wavelength (656 nanometres.) The bandwidth of a filter tells you how narrow a section of the spectrum it will pass. If you had a 12nm Ha filter then it would pass light with a wavelength between 650nm and 662nm. As well as the Ha, it will pass light which is not from Ha emission but which is close in wavelength. By comparison, a 6nm filter will pass light with a wavelength between 653nm and 659nm. You'll still get some non-Ha light, but less than with the wider filter.
Generally, the narrower the better. Unfortunately, narrower filters are usually more expensive. The one caveat is that for very fast optical systems narrow filters may cause problems due to bandwidth shift, but this won't be an issue for you at f/6.
The bandwidth of the filter won't affect exposure time appreciably. If you get a shorter exposure time with a wider filter it's because you're collecting light that's not part of the signal you want.
Cheers,
Rick.
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