In the 2 1/2 years now I've owned an M8 with an assortment of filters, I've proved to myself what I suspected back at the beginning just looking at the filters under a light and looking at how much/little surface reflection they have: the Heliopan IR filters are quite a bit better at avoiding ghost reflections and flare that washes out contrast. I noticed it especially with my 50 Summilux, because that lens I tend to use in harsh lighting situations where there are point light sources, like bulbs, candles, etc. Originally I had the Leica E43 UVIR and got many ghost reflections. The Heliopan 43 filter wouldn't let me reattach the Summilux hood, and yet it produced much less reflection. Finally I took the filter glass out and put it into a Leitz mount...actually had to shave the periphery of the Heliopan glass to do it.
If I had a noctilux and were going to shoot where there are bright point light sources I would get the Heliopan filter.
BTW I have not seen any objectionable color difference between them.