I also have considerable experience with both 6x6 and 6x7. I used Hasselblad cameras
for many years before the Pentax came out and found them to do a good job. However when cropping a negative/positive image to fit an 8x10 horizontal or vertical format you are not getting a great deal more actual image area than with 35mm. A full frame 6x6 reproduces and prints very well, but you do lose some of the "quality" you are trying to gain by using a larger negative. My choice over 6x6 is the Pentax 6x7. Before the "mirror lock up" was available I had no problem what so ever with the so called "mirror shake problem. Having spent much time using both formats, for me the 6x7 is a good way to go regardless of what camera you choose. 6x6 is a little better than 35mm. but 6x7 is a bunch better! I have found the Pentax lenses to be among the best I have ever had the opportunity to use. Nothing is lost to cropping with 6x7, unless you want to.