I think the OP is a fair assessment of the differences between Acela Business and Regional Business.
Since I am usually traveling between PHL and NWK or NYP, I have found the Regional business class to be a good choice since it is usually not crowded at all and if the train is on time there isn't a significant travel time difference between the Regional and the Acela for these city pairs.
I recently took the Acela FC between PHL and BOS and it was totally sold out between NYP and BOS. In fact, there had been some kind of ticketing snafu and it was oversold by 1 person. FC on the Acela is very nice though.

By the way, Select members receive three upgrade coupons (1 hour) and three club passes.