Originally posted by soppy Well the Hoya HD's are supposed to be outstanding
I don't think they were around when I purchased the HMC - I don't remember seeing them. And since I've been very happy with the B+W, I haven't looked for anything else.
Regarding IQ degradation, I've never seen any as a result of using a good quality filter. The main risk is flare, so if you take night shots and see issues with flare or ghosting, try removing the filter. But otherwise, there's no negative impact, or to put it differently, there's no practically significant negative impact.
Here's a nice article related to the impact on IQ made by bad stuff at the front of a lens:
LensRentals.com - Front Element Scratches
The DA40 is a tiny lens and I would avoid using a filter with it just to keep the lens package small. But I wouldn't be concerned of a filter affecting its IQ, regardless of filter brand, as long as it's relatively high end.