I don't know how comparable the B&W diamond tweeter is to the Accuton or Seas diamond domes, but ID companies don't typically produce their own drivers, and have to source them from OEMs whereas a large retail brand may be more vertically integrated with their own in-house drivers. While they certainly get better pricing than the average DIY'er on Madisound, the ID company has less control over the pricing of drivers since they don't control production.
I would say the Seas tweeter in particular seems very interesting. I've never seen a tweeter with usable output to 350Hz (!). A driver like that would make it pretty flexible I imagine for pairing with a mid or woofer.