Which camera will likely have "more" video specs? The R5, if for no other reasonthan higher resolution mathematically affords it capabilities of 8K, which has been rumored to some extent.
Which camera seems more natively attuned to producing video? The R6 as it shares the same sensor as the 1DX3 (so far as we can tell) but we are NOT yet sure about what CPU it will employ. If we assume the Digic X also like the 1DX3, then this camera should be able to produce the same video specs as the 1DX3 including RAW 4k and RAW 5.5K sans DPAF when in RAW and/or FF mode. That said, the CROP mode for 4k on this camera would also be at 1.3x like the DX2 and DX3, which is a very negligible crop.
This would further beg the question, what sort of 4K or 1080p modes will/can we get from the 45MP sensor of the R5? That's a LOT of data to sling around, so yeah maybe 4K120 but it will come with a major crop north of 2x.
So if you are mostly shooting at 24 or 30 fps for video, then an R6 will do a splendid job, particularly if you outboard it to an Atomos Ninja V with 10 Bit 422 CLog gamma and record directly to ProRes 422 or ProResHQ.
The 1DX3 tops out at 4K60 on that 20MP sensor with a FF readout with the Digic X CPU. Can the 45MP R5 do a true FF readout with all those same modes? I wouldn't think so. Maybe some cropping. Maybe some line skipping. We dont really know yet. If it can read out the whole sensor at 4k60, then that Digic X is far more capable than the 1DX3 is letting on... which MAY be the case if it can do the same 20fps stills via electronic shutter that the 1DX3 can with less than half the pixel count.
I can say for ME that as of this moment with what limited info we have, I'm in the R6 camp as I don't need another stills centric camera. I'm much more interested in maximizing FF readout for video across the widest framerates and settings possible and that is much more likely with a 20MP sensor than a 45MP sensor.
If massive oversampling was truly yielding significantly better results, then you'd see high MP cinema cameras. The new C500Mk2 is a 20MP very similar in size to the DX3/R6 that shoots 6K. Only time you'll see 40MP in a cinema camera is if they are building a full blown 8K model