I was facing the same choice.
The nature of my reportage style often requires me to take both stills and video captures of the same moment. On R5 I can shoot stills with shutter button and video with record button without any additional mode changes. With a CF card even after shooting bursts the delay between stills and video is <1 sec. On R5C you cannot shoot any video in the stills mode, and in video mode you can only take jpg screen grabs with the resolution that is currently set for video. Switching between the two modes on R5C requires a reboot which takes 6-10 seconds. That was the ultimate deal breaker for me.
R5C uses cinema OS in video mode, which is rolling full sensor readout even when you are not recording. This means that battery drain is constant as if you were shooting video. So as soon as you turn the camera on with cinema OS, the timer starts ticking and you have something like 30 min in 8K mode, 1h in 4K mode and 2h in 1080p depending on the res you set in the menu. So you will need a lot of batteries. R5, on the other hand, uses regular binned sensor readout as in photo mode until you start actually recording.
I was thinking that I could live without IBIS and use gimbal/monopod, but IBIS proved to be so good that I rarely use other stabilizing gear.
I've been shooting with R5 for over a year and realized quite early on that my choice was absolutely correct and thankfully I did not go for the R5c.
I was leaning towards R5c initially for two reasons: 8K60p and cooling fan. In reality, I never shoot in 8K, apart from a few initial tests, in part due to enormous file size, and in part due to the fact that for video resolution is not as important and 4K is already plenty. The lack of a cooling fan on R5 also proved to be a non-issue since the firmware updates, as I have never seen the warning come up. On the other hand, with R5 I can shoot all day with 1-2 batteries, because standby time does not drain much power. With R5c I would need something like 8 batteries, or an external pack.