Soeren, I started still photography with a Nikkormat, eventually got a Retina to use as a cycling camera. The Retina wouldn't hurt me as badly if I spilled and landed on it and would cost less to replace.
I moved up to 2x3 (you'd say 6x9) when I'd taken enough wild flower shots that weren't fully satisfactory. On 35 mm, my choices were a close-up of the flower that excluded its setting but showed the fine details or a shot of the flower in its setting with the fine details poorly resolved. 2x3 solved that, more or less. In recent years I've shot mainly 2x3.
Most recently I've moved a little up to 6x12, haven't shot enough yet to be sure that what I gained is worth the expense and effort.
In cine I settled on Super 8 and ran through a number of cameras. Used S8 cameras aren't as reliable as one could want, so one needs a main camera and a back-up for when the main camera dies. I replaced the second cameras, one at a time, with nicer and nicer ..., eventually ended up with a nice kit. For my budget and preferences S8 was the only format that made much sense. 8/8 had no advantages, offered worse image quality because the regular 8 frame is smaller than the S8. Larger formats want more expensive film and larger heavier everything.