I don't obsess over it. I find that I get much better quality results much easier with medium format. I can get good print quality out of 35mm but it takes a lot more effort. So I actually do what you say - I use what gets me the shot, but I tend to use the camera that gives me the biggest possible negative while also getting the shot. The bigger negative gives me quality much easier, more options for cropping etc.
Originally Posted by Thomas Bertilsson
I do shoot some low light B&W in 35mm because I have faster lenses, and I shoot moving subjects more often - even though I have the AE prism for my M645 Pro, the 35mm is still faster handling with better lens selection. Sure print quality is secondary to getting the shot, but everything else is secondary to getting the shot. If you can't get it with MF, use 35mm of course - or Minox or digital or whatever. But if you can get it with either then other preferences come in to play.