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A lower bandpass results in a darker background ADU count so has the ability to pick out the fainter signal of the emission line. This would probably be lost when trying to combine the data. You could get tricky and use pixel math to favour certain characteristics of an image, but I doubt the result would be worth it.
A lower bandpass is more effective at cutting through light pollution if imaging from the suburbs, but in reality a moderate bandpass is as good. Obviously this also assists in increasing the time before you hit the sky glow limit. Many opt for 3nm SII and OIII filters to maximise the signal from these emission lines. Anyone who's done narrowband imaging will note that getting good SII data can be a challenge. HA on the other hand is much easier to pick out and get good signal on. Would suggest getting a moderate to low bandpass but be prepared for long exposure times.
Cheers
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