I know both Mount Airy and Bob's Bikes well. Mount Airy has an insanely huge collection of bikes of all stripes, and is a real regional resource (especially for those of us who like oddball bikes.) Bob's is tiny, but Bob has been working with Bionx longer -- and he is willing to dedicate much more time to trying to get your Bionx install exactly right. I can't vouch for his business model, but I bought my Bionx setup from him too.

Charlie
http://ElectricCyclist.com