When I 'got into' minis(after nearly 40 years involved in 'big' horses) in 1984, there WAS NO B division in AMHR; they did not institute that until several years later. So, of course, I had AMHAs...and I took pains to choose those to show that were GENUINELY 34" or less--though way back then, Foundation Oversize was in effect, and I bought one mare who fit there, and moved another into that division who came to me w/ 'regular' AMHA papers, but did measure nearly 35"...some of us do believe that rules are to be followed,and act accordingly.
At the time, I felt that instituting the AMHR B division was largely a financial move, to increase income...and honestly, that is still my opinion about their original reason. That said, I have come to believe that it became 'worth' more than just increasing income...and for myself, for driving, I have come to prefer a nearer-38" horse. I am not tiny, and like to plesure drive w/ driving clubs and the like, so 'more power' is helpful. I still have and love, several of my 34" and under horses, some of which I bred and/or trained, and/or showed; the very first mini mare I chose to buy was a 'Sooner States' mare who is EXACTLY and honest 34", who is still with me at 29 1/2, and produced excellent offspring---she did well by me, and I intend to do well by her until she is gone. However, my one B sized is an unregistered, very 'UK Shetland' looking, gelding. I did not need registration for what I want him for; his driving training and abilities are his 'ticket' to desirability, IMO.
For loving on, and general 'easy to have around-ness', I hope to never be without at least one 'A" mini. For serious driving of a single horse, give me a larger 'B' mini...there are certainly good niches for both!
Both registries have their pluses and minuses, IMO, but should be supported IF you are actively involved. I no longer have much if any desire to breed show(unless it's the occasional Halter Obst. or Driven Obstacle w/ my 34" AMHA Champion, now 18 YO, homebred/trained gelding, who happens to be out of the above-described mare)...been there, done that, got my fill of it...but am still hopeful that each will continue to grow but IMPROVE as they go.
Margo