With regards to the ETRS and SQ sizes, I have had both and the ETRS feels much smaller and compact in hand compared to the SQ.
Anyway, the SQB is a nice camera and I had the 80PSB that goes with it as a kit. Sharp, easy to use, easy to load, generally a good camera for a budget. As said, finding it as a complete kit is easier and works out cheaper. The only thing you should look out for is the condition of the focusing screen as they tend to get scratched a lot. The other advantage of the SQB is that it is the newest model (went into production in 1996) in the SQ line so you know you're not buying an ancient camera. It won't take a motordrive or any of the metered prisms though.
All you ever wanted to know about the Bronica line: