I doubt you will go wrong with any of the modern Bronicas (ETRSi, SQ-A, GS-1, or the RF-645 provided you like portrait orientation). It should probably depend on the aspect ratio and negative size you prefer. I chose my ETRSi, which I am very happy with, for the following reasons:
1. I enjoy hiking and it is remarkably compact for an MF system. With the waist-level finder and 40mm lens it takes up very little room in my pack.
2. Almost all the lenses from 40mm to 200mm take the same 62mm screw-in filters. Since I mostly shoot B&W landscapes, and use a variety of colour filters for contrast control, it's a real pleasure to only need one of each colour! I considered the SQ-A but the 40mm takes a great big 95mm filter and the 50mm PS takes a 77mm filter while the rest of the lenses take 67mm filters. The GS-1 wide-angle 50mm lens also takes 95mm filters.
3. It's the least expensive of the modern Bronicas, and lenses etc seem to be widely available, and there's also a wide selection of focal lengths (compared especially to the GS-1). I believe the ETRSi's PE lenses are as good as the GS-1's PG lenses.
4. I can live with the negative size compromise (6x4.5 vs 6x7 for the GS-1) as I don't print huge. I have almost convinced myself of this.
Andrew