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Paul,
What do you mean that the Astronomik Ha filter appears to have a shift to the blue? All Ha filters are centered on the Ha emission line 656.3nm. The bandpass size simply implies how many nm on each side of this line i.e. a 6nm filter has a window from 653.3nm to 659.3nm. It hasn’t shifted the emission line centre. The 13nm simply has a larger window and as such lets in more continuum energy. The only exception to this are companion filters that are slightly offset from the narrowband emission line. Companion filters allow you to subtract the continuum energy from the narrowband data resulting in a pure emission line i.e no stars, zip, nothing; just Ha nebulosity (can’t get anything purer).
I haven’t personally used an Astronomik Ha filter, but can speak about the Custom Scientific or Astrodon filters. Given your light polluted environment, I’d go with a 6nm. You’ll need longer exposures than a 13nm (for obvious bandpass window reasons), but I suspect it would deliver a better result – certainly in the background ADU readings anyway.
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