How bad (or negligible) is your light pollution? Will you be imaging regularly in moonlight? These are the things to really consider. Narrower bands will reduce extraneous inputs, at the cost of longer integration for the same SNR. I think 5 (or 7) is a pretty good compromise - 3nm is for seriously constrained NB! Heck, I image at 12nm (
http://www.astronomik.com/en/photogr...cd-filter.html etc.), but I am in dark skies.