Let's see. This is a broad guess and has no substantiated claim in hard fact (so don't bash me, bonechilling), but Bridgestone is a large company, and Nagasawa is a smaller one, so possibly Nagasawa puts a little more time into hand-building their frames?
Nagasawa is also considered, along with 3rensho, one of the best keirin frame builders.
I've owned a Bridgestone NJS, it was a mighty fine frame, no complaints. Nice track geometry, nice lugs, nice feel.
Kaisei makes some nice tubing, I'm a fan of it. The Kaisei 8630 is the best, though, and as soon as a frame goes up on keirinculture or eBay with that stuff, I'm keen on getting it for myself.