I love my SQ-A and the only other 6x6 SLR I own is the much older Bronica C. It's great if you want to go old school, but the SQ series is all around a better camera.
If you want to look into 6x7 there are more options. I had three 6x7 SLRs at one point, the Bronica GS1, the Pentax 67 and the Mamiya RB Pro SD. I ended up selling the RB because it didn't get enough use. I use the Pentax for landscape and the GS1 for handheld. The RB is a very capable camera in all regards but the design, size and weight make it most suited to studio work, which I don't do much.
Of course if you are willing to go with a rangefinder and can't decide which format you like best the Fuji GF670 is the bomb!! It's rather expensive but I absolutely love mine.
If you want a small, portable 6x6 rangefinder with exchangeable lenses I would strongly recommend the Mamiya 6. It has lots and lots of strengths, and very few drawbacks. Once again, not cheap, but you definitely get what you pay for.