First on Acela is not really worth it, IMO--I do it only when I have an upgrade coupon and want to eat on the train. I sit in Business (a.k.a. coach) on Acela and that's just fine.
Depending on what time of day it is, your Acela will either be packed or about 2/3 full. They tend to sell out at peak times.
I have never tried Business on the regional train, but I don't mind taking a regional in coach. In the middle of the day, Mon-Thurs, these trains tend to be emptier than the Acela. They are pretty comfortable, and a lot cheaper. No footrests; dingier bathrooms; no arm rest between the seats.