I had both and eventually sold both (consolidated to Leica M and Leica SL). However, I would pick the Q3 over the GFX100RF unless you really like having a lot of megapixels and detail. Better lens, more responsive, some form of stabilization, somewhat better AF, better design, and the list goes on.
Lack of IS is the main drawback of the GFX RF. And the simply awful hood/filter system. Other than that, it's a really fun camera to use. I think the lens, while a bit slow, is excellent and compares very favourably to the Q3-28. Basically, it's a somewhat bigger X100 without IS.
Personally, I find the Q3 more useful because of its size and OIS, BUT I see how some would really have some fun with the GFX RF more than the Q3. I'm keeping both.
Unfortunately, I don't think it's a decision you can make based on logic. Go with your heart.
I think you already answered the question - not using the Q3 - wanted to get into Fuji.............
A camera just sitting around is a paperweight. Any camera that makes you want to shoot is a great thing. I would not do the trade but I'm not wanting to get into Fuji. You can always sell the fuji and get another Q3 anytime. Why not give it a whirl?
Get your hands on one ... rent, beg, borrow, demo, etc.
To Gordon's point about going with your heart ... getting one in your hand is the path to your heart. They'll either connect readily, or they won't.
Technically, the matter of IBIS is already a known. Kinda like shooting with an M means, no IBIS. Folks have dealt without IBIS far longer than IBIS has been around (can you say film, etc.). If IBIS is a hard requirement then one likely isn't looking at an M, nor the GFX RF. Hmmm, neither RF has IBIS, wonder why that is.
Given the DR of the GFX, a bump or two in ISO to keep your shutter speed at 1:1 (1/30s) or faster territory isn't a big deal. Want to go around doing hand holding of 1 second, then IBIS is your tool. But, you weren't going to do that with the Q3 OIS anyway, so that's a wash.
Bottom line, find one to try, before you buy (if you can). If it puts smile on your face when using it If it keeps a smile on your face when processing it.
Never used Q3 but love GFX100RF. Two big problem with RF: power switch can be easily tripped. Shutter is very mushy. Other than that, it is one of most fun camera I ever used. Control dial and button is always perfect for me to have everything in reach.
Slow lens force me not rely on shallow DOF to make photo more interesting but content. I guess using it is more like using a old film camera like Mamiya 6/7, Fuji GA, GW.
IQ is as good as it can be. 100M sensor is superb for me to crop.
Fuji's 100RF gain a lot of my respect for Fuji as a camera manufacture. I don;t have the same feeling with GFX100II though, I use GFX100x mainly use it as tool that just deliver my need. But for RF, I don't really see that in others and hard to put word into it. I mean some manufactures offer very strong tools that practical and powerful can be used for anything such as Canon, Nikon Sony and others focused a lot to premium build, luxury feeling, simple UI, fancy marketing like Leica, Hasselblad.
This RF made me think about another layer of feeling of taking picture, it is not about made your life much easier or offer on your face type of premium luxury feeling but in a much subtle way, made you slowly love it more and more with more use. IT is a camera can grow with you if that make sense almost like a film camera. I can image be forced just use it and be happy with it to focus on photography and maximized the output of it.
Have both but the Q3 43. Depends on whether you crop or not. The Q3 43 cropped to 90mm will have higher resolution around 15mp I think, than the GFX100RF’s crop to 90mm FF equivalent. Carrying both provides a versatile set that covers a good range for the travel landscapes for my use case.
If it keeps a smile on your face when processing it. 