The price differential likely comes from the use of coatings (MRC, anti-reflective) on the B+W with no mention of any coatings on the Tiffen, use of brass in the ring (less likely to seize up when trying to remove the filter from a lens or another stacked filter),and also some brand-name cost baked into the B+W as they are the premium brand.
::Edit::
I've used the B+W brand filters primarily for my ND needs but not in the smaller size you're looking to buy. I initially had used some lower end ND filters which tended to produce some reflections that got picked up whenever I had the sun in certain angles to the camera. I eventually bought my B+W ones; though in retrospect, I could have gone with some lower premium brands that have similar coatings technologies and saved a little.