If I were getting the Bronica SQa/ai/am I would definitely choose the SQAi. The 'i" in "Ai" stands for "improved" in that it is the latest model of the "SQ" line. It has mirror lockup for mirror up shake free time exposures. That would come in handy for any time you want to take a landscape photo with your lens stopped way down for that "forever" depth of field.
As far as the SQAm is concerned, if your camera battery were to go low, you won't be able to continue by manualy advancing your film because there is no hand crank to further advance film and cock the shutter.
All cameras have their pros and cons.
Talking about the Pentax line, lack of changing film backs is a con...
You say you're not interested in changing filmbacks now, but there will come a time when you'll wish you could change from B&W to color in mid roll.
Perhaps to take the same shot in both mediums.
So weigh pros and cons of each of your contenders. When you do that, the "pro's" have it.
ps: I have just bought into Bronica ETRsi recently (still building my kit).