I've owned a B7800, B3200 and currently own a B2320 so I've had some actual experience over time and use with some of the different B models but I'll still tell you to go to the Kubota Manuf site and compare the differences on paper. The B2320 is the lowest current HP B and the B3300 is the current most HP B. I'm sure there is a weight difference and frame size difference between the 2 ends. The facts will show if the B2320/B2620/B2920 are all the same except for HP and then the B3200/B3300 if they are on a bigger frame. Look at the weight. Kubota Tractor Corporation
B2320, B2620, B2920 (known as the "20" series) are the entry level basic B tractors. Difference between them is basically the hp.
B2630, B3030, B3000 are larger framed and premium tractors with more standard features like position control, better seats, extendable 3PH links, etc. The B3000 and older B3030 are available with cabs.
B3200 and B3300 are again basic B tractors but larger framed and more hp. The B3200 has a mid PTO and can run a mid mount mower or front snowblower. The B3300 does not have a mid PTO.