6 x 6 is my favorite format for 120 because it gives you a unique look. 6 x 9 gives me the ordinary 2 by 3 look I already get from my digital images - which I often crop to square format. I recently got the Lomo LC-A 120 and one just can't "crop" or "post" the look of these pictures. Of course it also depends on your subject matter... I also find 6 x 12 very special, but then you need a special subject for this. It is very nice not only for landscapes but also (in vertical orientation) for architectural subjects, where one get's something like one would get with a tilt-shift-lens, but again with a very special look. I'm using the Lomography Belair with their glass lenses, which are kind of sharp even in the corners.