I've owned/own both and even though the current BX25D-1 has a manuf defect of the tractor continuing to roll on when the pedal is released I'd buy the BX before the B if I used it 90% for mowing with the price difference your giving. If I could wait I'd wait until Kubota fixes the BX25D and then buy one.
The BX is a better mower by quite a bit.The B is a bit more capable at moving dirt.I have owned two BX's and a B2620.I found the BX to be a more comfortable machine to operate.
The B2320 looks like almost straight MSRP from the Build My Kubota application, assuming 1 year extended warranty.
Maybe the dealer is including more than what I have assumed?