Viewfinder
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Viewfinder

...the GR III does not have a built-in viewfinder, whereas the X100F does. And in fact, the X100F's viewfinder is one of its best features. Unique to Fujifilm, the X100F features a 'hybrid' finder which can be switched between a high-resolution electronic view, and an optical view with exposure and focus information overlaid. Personally, I use the X100F almost exclusively in EVF mode, but a lot of photographers swear by the immediacy of the optical view. Impressively, the X100F can offer a version of the classic rangefinder focus aid by overlaying a portion of the live view feed in the optical finder. It's hard to describe in words, but works well if you're a fan of manual focus.

With the GR III, on the other hand, you don't have the same options. While simple optical finders can be attached (and Ricoh will sell you one, for quite a lot of money) you miss out on in-view focus or exposure information, and there's no option to add an external EVF.

Verdict: Fujifilm X100F's built-in hybrid optical / electronic wins by a mile.