The comments by Loose Gravel and OleTj are right on the mark. I started my photography using medium and large format and avoided 35mm for many years on the issue of grain and print quality. When I finally started to use 35mm about 10 years ago I realised just what I had missed all those years and now shoot mostly in 35mm because I have grown to love grain. I still have and use 4 x 5, 6 x 6, 6 x 4.5 and even an old whole plate camera with lens that have no shutter so it's caps off and wait around for hours when the lens is stopped down to f125.
Back to your dilema, and the issue of 6 x 4.5 against 6 x 9. I went through a similar dilema a number of years ago when I upgraded to a single lens medium format from a fixed lens 6 x 6, Minolta Autocord a dear old friend that I still have. On the grounds of weight and cost I finally decided on a Mamiya 645 Pro and have never regretted that decision. I have picked up a full range of second hand lens and several backs at every reasonable cost. On the issue of quality, you need to sort out your film exposure and development details and you will, for I know you are methodical and determined. Prints made from my 6 x 4.5 negatives have often been mistaken for 4 x 5 negs so there is no need to worry about loss of qyality.