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).