It really depends on what your goals are. I don't have any experience with FMHA but from what I've seen they are more of the old style mini look.
For me personally, I like to drive and I like refined, huge moving horses with typey heads and swan-like necks. The American Shetlands have that the most but it's hard (not impossible but very hard) to find that in a true under 34" size and besides that I like a bit more height and power for my driving horses so that they can be CDE prospects as well. So that is why I prefer ASPC/AMHR horses.
Some people like breeding for halter only or therapy and they love the 34" and under horses. Or they just enjoy driving smaller horses because of their size. Equipment is also a factor as to be properly fitting you are going to need two different harnesses and/or carts for your 32" driving horse vs your 38" driving horse. It can be as simple as that or the fact that there are more R shows than A shows in their area.
Other people like stock type and that seems easier to find in under 34". Healthwise, Shetlands have insanely high metabolisms and it's nearly impossible for them to founder on grass unlike some of the smaller, older miniature lines. Shetlands are also less prone to the cresty necks and dwarfism is not a worry. Of course all sizes can have genetic hoof, teeth, or joint ailments, but that is where you make sure you are buying from a breeder who truly breeds for soundness! Temperament is very much genetic as well and the same factors apply. You have hot lines and laid back lines in all sizes as well.
The most important thing to do is to try to outline your goals before you start building a herd and make sure you only buy what fits your goals. See what lines are producing the types that you like. So many people try to breed multiple styles, types, etc, and the most successful breeders are those who focus on one specific type and only have the lines that produce it. Consistency is key. Their horses go on to suceed in everything from performance to driving and halter because they are well bred for brains, soundness and beauty and the results are the same every time.
