I've owned both. There is really only two things of real note. Oh sure, two speed vs three speed, shrug. Not a biggie. The dash gauges on the B are far superior to the BX, but again, not life or death. Both use 1/4" 3 pt valving, while the B works better, again, not a biggie, imho. The list goes on. But here are two genuine differences and you'll have to decide for yourself.
The BX has a few things that are a fact of it's diminutive stature. Ground clearance is about half the B, if that matters to you. On most things, it matters not, but where ground clearance is required, there's no making the BX taller.
The 3 pt width and height is also limiting. Kubota doesn't call it "limited Cat 1" like Deere does, but it is. Again, can't make the BX wider or taller. Some implements need more ground clearance. The good news is that XB or properly sized versions of most implements are sold everywhere. They make life easier in lots of ways.
Finally, the B does not have the plastic trans cooling fan, but the BX does. Hopefully, Kubota will follow Deere's new 1000 series lead and get rid of that gosh awful thing on the next generation of BX.
Lest you think I'm ragging on the BX, not so!! Sweetest bugger ever!! Just have you eyes wide open. The series are different for a reason.