I own everything from half frame to 8x10. I try to use the best tool for the job. But, sometimes, I try to come up with the best job for the tool. I'll try to figure out what a specific format can do which others can not (at least as easily). As an example, multiple half frame images can be printed together in a 4x5 (or 5x7) negative carrier. I'll shoot consecutive frames I want printed as a single image. Here's an example (it's 4 consecutive half frame exposures, printed on watercolor paper and hand painted):
Lately, though, I've been using my two Fuji 6x9's the most, but they're the newest. I really like the 6x9 negative.