The SQ-Ai has some revised electronics and it uses 4 button batteries, while the SQ/SQ-A/SQ-B use a single 6V PX28 or 4LR44 battery (commonly available, same as in the Canon AE-1 etc). The SQ-Ai needs a plastic button battery holder which is commonly lost. I have designed a 3D-printable replacement with files available at https://www.thingiverse.com/thing:5256045
The SQ-Ai has some different capabilities WRT motor drive and dedicated TTL flash that are probably not that important to most users today.
I don't think there's much to choose from in the electronics. The electronics in these cameras, as in all the Bronica ETR/SQ/GS cameras, are for shutter timing and metering, and not really that complex.
The leaf shutters and mirror mechanism are completely different in the ETR/SQ/GS series versus the S2/EC focal plane shutter and mirror (note the EC is also electronically timed).
The SQ-Ai has some different capabilities WRT motor drive and dedicated TTL flash that are probably not that important to most users today.
I don't think there's much to choose from in the electronics. The electronics in these cameras, as in all the Bronica ETR/SQ/GS cameras, are for shutter timing and metering, and not really that complex.
The leaf shutters and mirror mechanism are completely different in the ETR/SQ/GS series versus the S2/EC focal plane shutter and mirror (note the EC is also electronically timed).
) It might be of some concern that some of the former repair guys seem to have retired and some parts may be more difficult to find. Of course for casual use, any given unit could potentially work for years.