Not to have an argument or anything like that, but I used to print in commercial labs for years, 8x10 negs/chromes (cibachrome), 4x5, 6x6, 6x4.5, 35mm and I can tell you that the 6x6 was always sharper and had more contrast than the 6x4.5 version. I am not naming brands but I am naming film sizes. Every time we came across well-exposed chromes/BW, negs the 6x6 was always sharper and more contrast than the 6x6. Now the 6x7 format also produces excellent results as well.
I am just speaking from printing experience and looking at thousands and thousands of slides of different formats. This is why years ago I went with the 6x6 instead of the 6x4.5 format because of the picture quality the 6x6 and above produces.
I based my decision back them on image quality and print quality. Then before I made the purchase I rented the different formats and printed them from 8x10 up to 30x40 for comparison and again the 6x6 and 6x7 formats prevailed every time in both sharpness and contrast.
I really liked the 6x4.5 at the time but what I saw convinced me otherwise.
Just my two cents.