Full Frame all the way, every day. I still use my trusty old Canon MkIV.
Typically other problems are more relevant than the main body of a camera. Do you already own gear from that brand or another? Will existing lenses fit? What about the ecosystem? Sony has a wide range of lenses whereas Fuji hasn’t, and 3rd party lenses are rare too.
Similarly, try the two cameras in the shop, the menu system can make a huge difference. I really like all the manual buttons on the Fuji with easy access to the most used functions, lovely when you shoot mainly, or a lot in manual mode (like me).
After that ask yourself what are the features YOU NEED and which of these features each camera has, for example 4k @ 60Hz.
Full frame tends to have better dynamic range and better noise than the APS-C counterparts. So is low light performance important? Autofocus is another big one, but depends on your preference. If you want to use the camera mainly for product photography, for example, you wont need super accurate AF. If you wanna shoot fast moving objects it would be useful.The XT4 comes with 20fps thats pretty nutty. Weirdly enough the Sony seems to have more focus points, which typically indicates better AF.
Back to the use case, if you like to do super wide angle landscape or street photography a FF sensor is better as you get better use out of your, hopefully FF certified, lenses. Especially for wideangle a FF is a god sent and you need the fitting lenses. Chromatic aberration, darkening in the corners and so on are very common for APS-C lenses on FF bodies.