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As Barry said, the narrower filters will reject more of the sky glow. This will give you better contrast and won't really need much longer exposure time - the light that you're rejecting isn't signal that you want anyway.
With Ha, 5nm is a good compromise because it will give you NII as well as Ha and that will help with a lot of objects (e.g. the Helix Nebula is significantly fainter with a 3nm Ha filter.)
You didn't mention whether you were considering other narrowband filters as well as Ha? Assuming you are: Moonlight will affect OIII more than Ha, so if you want to image with the moon up a 3nm OIII filter is a good investment. If you're getting SII as well then a 5nm is adequate.
The other filter parameter that is important is transmittance. This tells you how much of the light you're trying to capture gets through the filter. A filter with 90% transmittance will give you quicker results than one with 70%.
Oh, and narrow filters won't work so well with very fast optical systems but unless you're near f/3 or faster it's not a problem.
Sorry, don't know of anyone that carries Astrodon filters locally. I've always ordered mine direct from the US.
Cheers,
Rick.
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