Wirelessly posted (BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
Business Class is the lowest class offered on the Acela. Seating is always 2 x 2.
Business Class is the highest class offered on the Regional. On at least some trains, the Regional Business Class is half of the Café Car, and features 1 x 2 seating.
As to which will be more "crowded," that will probably vary from train to train.