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It's all about bandwidth. A wider bandwidth won't dim the stars as much,but will provide somewhat less contrast enhancement for the nebula. In terms of bandwidth, the 3 filters mentioned can be ranked from wider to narrower:
Astronomik, Lumicon, DGM.
Any of the 3 would be better than no filter at all. Users of small scopes might find the Astronomik more pleasant to use, and I appreciate the H-Alpha transmission, but the other two filters do enhance the nebular visibility a little more.
Whichever one you get, you'll find the contrast enhancement incredible. The Lagoon nebula (M8), for example, seems to quadruple in size in my scopes when using a UHC filter.
I don't recommend UHC filters with too-wide bandwidths (such as Celestron/Baader/TeleVue/UHC-E) unless you are looking for a replacement for a broadband filter that does more than a simple broadband can do.
Don
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