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As Suavi says, the narrower the bandwidth the better contrast you'll get (better at rejecting everything other than the Ha signal). In some cases the 5nm will give a better signal than the 3nm as the 5nm contains both the Ha and NII signal; the 3nm is narrow enough to split the Ha and NII.
If you already own LRGB Astrodons I would suggest keeping with Astrodon in an attempt to remain par focal. Astrodon filters are 3mm thick which I think is thicker than most others.
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