It's really a question of testing 'em. Can anyone look at 2 16x20 prints and tell which was the B&W filter and which was the Tiffen?
When the DSLRs-for-video thing really got going, I did a lot of lens and filter testing (since I use Nikon for much of my 35mm film stuff, it was easy to test my gear on DSLR bodies). I learned a lot about sweet spots for various lens apertures, but also found that even the cheapest NDs - which I bought for initially experimenting with digital video - were great for sharpness and only affected color at extremes - 3 stops and over.
Since I shoot stills and video at all sorts of events, I have to change my skylights yearly - they get dirrrrty. I found Tiffen and Marumi to be fine and they save me a bundle.
In fact, many Marumi filters test very, very well these days. But if you're really concerned, shoot some test film (with filter and without), setup your enlarger for a 20x24, and run some 8x10's of the centers and edges. You'll know in an hour or so.