I have all three (ETRSi, SQ-B, GS-1) and each is magnificent in its own way. And with the accessory Speed Grips the cameras are very hand-holdable, with a convenient hot shoe for studio work. Which one to buy depends on what format best suits you; all three are equal in construction and optics. But from a practical standpoint, the ETR series has the best availability of system components on the used market. SQ series gear is a little tighter but still good. As much as I love my GS-1 (one of the best cameras I've used), I don't think I'd invest in one today. Focusing screens and other accessories have become very difficult to find. Even things like 120 film inserts and some lenses can require frequent checks with eBay/KEH to find.