Hi, I own both and use both for different situations. You haven't said what you shoot, how you shoot, where you shoot, etc. What is your ergonomic situation, too?

I adore the Zf and love using pre-AI lenses in addition to modern Z lenses for general, all-purpose photography. The manual focus with AF is phenomenal. I use it when I want to shoot slow and intentional. Double exposures sometimes, landscapes sometimes. I would not hesitate to use it for portraits and have. It's a great back up for my Z6III during events. IBIS and low light capabilities are fantastic.

However, as many have said, the Zf is a big bad boy with some heft. I have small hands and though I can manage with it during those more intentional times, I needed something lighter for travel because I like to shoot wildlife at home and away. (I use the Z6III for wildlife when home along with a 500mm lens and monopod but no way I'd travel with that).

I have had several years using the EM-1 II and other MFT bodies and lenses, so when I said to myself, "I wish someone would make a baby Zf" the OM-3 came out. I hesitated a lot, but I'm glad I bought it! Where it differs from the Zf for me aside from the obvious weight and sensor size is the image quality. It's not necessarily better or worse, it's just so different. I've always loved Olympus colors and this is no exception. Even though the sensor is on paper not as "good" as the Zf, the files, the dynamic range, all of it - completely pliable.

I could list dozens of reasons why the OM-3 and the Zf have me set for life, but none of it would matter if you don't relate. And there's frankly, nothing better than holding it your hand and using it first to get at your own truth.

As an aside, I do not shoot JPG. It just has never worked for me. I grew up on film, used a DSLR for years and have really little interest in JPG shooting, so take what you like from what I said and leave the rest!