[Klaus]

https://www.canon.com.au/camera-lenses/rf-14-35mm-f4-l-is-usm

MTFs here:
https://cweb.canon.jp/eos/rf/lineup/rf14-35-f4l/image/spec/spec-mtf.png

vs the 15-35mm f/2.8
https://cweb.canon.jp/eos/rf/lineup/rf15-35-f28l/image/spec/spec-mtf.png

[Brightcolours]

Those look pretty good.. But the price at USD $1700 is a bit higher than I anticipated.

[Klaus]

Yes, $1700 vs $2300 for the RF 15-35mm f/2.8 seems a bit strange.

If you can afford the 14-35mm you could as well go for the faster lens. A single extra mm can't really cut it.

The Sony 12-24/4G costs $1800 - so maybe they took this as guidance. The Canon is 2mm longer but with IS so $100 less isn't unreasonable.

Conversely ... the Sigma 14-28mm f/2.8 costs $1300 - as always a bargain.

[Rover]

Correction: the Sigma tops out at 24mm. It's a noticeable difference.
I would use an xx-35mm mostly as the only wideangle, the next lens being the 70-200, the 100-400, or at least the 45mm. Though I'm using the 24-85 sometimes, too. Point being, 24mm vs. 35mm at the long end is a pretty big deal. The ability to mount standard 77mm filters (though heaven only knows how big the vignetting is...) is as well.

[Klaus]

The I'm only using a wide-angle and tele zoom lens didn't work for me. I tried it but I ended changing lenses so frequently that it was an annoyance.

So far only the Panasonic 20-60mm seems to have the potential of preventing lens juggling sufficiently enough - but that's not a stellar lens (albeit probably Ok at 24mp).

[Rover]

Since getting the second body, it's become that much easier for me. :-) With the bodies of different formats, you're getting even more possible combos. I don't brandish two camera on every shoot but for the more dynamic and/or important ones, it's a great idea. If - in some other ideal world - I would've gone to the R system, I imagine using this 14-35 lens and a 70-200/4, though the lack of native, good and affordable intermediate option is a little concerning.

[Brightcolours]

Since getting the second body, it's become that much easier for me. :-) With the bodies of different formats, you're getting even more possible combos. I don't brandish two camera on every shoot but for the more dynamic and/or important ones, it's a great idea. If - in some other ideal world - I would've gone to the R system, I imagine using this 14-35 lens and a 70-200/4, though the lack of native, good and affordable intermediate option is a little concerning.

That 24-105mm coke bottle that Klaus is currently testing would run circles around the "standard zoom" you use on your current 1D, though. And it would be ok especially on an R6. Just don't forget to not turn the lens correction "off" ;-)
And of course, you can use any 50mm lens as intermediate between those two lenses too (even your 45mm f1.8 Tamron), and as lower light tool.

Just out of curiosity (and me forgetting), what is that second body you have?

[Rover]

Since getting the second body, it's become that much easier for me. :-) With the bodies of different formats, you're getting even more possible combos. I don't brandish two camera on every shoot but for the more dynamic and/or important ones, it's a great idea. If - in some other ideal world - I would've gone to the R system, I imagine using this 14-35 lens and a 70-200/4, though the lack of native, good and affordable intermediate option is a little concerning.

That 24-105mm coke bottle that Klaus is currently testing would run circles around the "standard zoom" you use on your current 1D, though. And it would be ok especially on an R6. Just don't forget to not turn the lens correction "off" ;-)
And of course, you can use any 50mm lens as intermediate between those two lenses too (even your 45mm f1.8 Tamron), and as lower light tool.

Just out of curiosity (and me forgetting), what is that second body you have?

Somehow I doubt a lens described by Klaus as a "coke bottle" would "run circles" around the 24-85 that I find completely sharp enough for work, if having noticeable CA at some focal lengths, plastic build and no IS.

My second body is the 80D; before that, I used a 650D (my wife's using it now with the 18-135 STM).

Sure the Tamron 45 could be adapted, but I spoke of the lack of "native" options like a good RF 50/1.4. Canon just can't get this FL/aperture combo right without drama, right? Somehow I doubt we'll be seeing it and with the apparent crackdown against Samyang lenses by Canon, I doubt we'd be seeing a third-party option either.

[Brightcolours]

Since getting the second body, it's become that much easier for me. :-) With the bodies of different formats, you're getting even more possible combos. I don't brandish two camera on every shoot but for the more dynamic and/or important ones, it's a great idea. If - in some other ideal world - I would've gone to the R system, I imagine using this 14-35 lens and a 70-200/4, though the lack of native, good and affordable intermediate option is a little concerning.

That 24-105mm coke bottle that Klaus is currently testing would run circles around the "standard zoom" you use on your current 1D, though. And it would be ok especially on an R6. Just don't forget to not turn the lens correction "off" ;-)
And of course, you can use any 50mm lens as intermediate between those two lenses too (even your 45mm f1.8 Tamron), and as lower light tool.

Just out of curiosity (and me forgetting), what is that second body you have?

Somehow I doubt a lens described by Klaus as a "coke bottle" would "run circles" around the 24-85 that I find completely sharp enough for work, if having noticeable CA at some focal lengths, plastic build and no IS.

My second body is the 80D; before that, I used a 650D (my wife's using it now with the 18-135 STM).

Sure the Tamron 45 could be adapted, but I spoke of the lack of "native" options like a good RF 50/1.4. Canon just can't get this FL/aperture combo right without drama, right? Somehow I doubt we'll be seeing it and with the apparent crackdown against Samyang lenses by Canon, I doubt we'd be seeing a third-party option either.

Klaus would not be impressed by that old 24-85mm Canon at all, on a modern body. The 24-105mm cheap one will beat it easily, that is how much lens design has progressed. 
I read that even the EF 28-105mm f3.5-4.5 II beats that 28-85mm on the borders (on FF). That lens was not all that sharp, but "pleasant", on APS-C 8-12mp cameras. Klaus only tested that 24-85mm on 8mp APS-C (350D). That was my first DSLR by the way.

So, yeah, I have no doubt that you would do quite ok with that cheap 24-105mm version, in general.
If you have that 45mm, why would you want a "native" one when using two bodies and seeing the advantage of using lenses on different formats (hello lens equivalence, my old friend) ?

[Rover]

Since I'm not using the 24-85 on FF, I don't have to worry about the outermost image field. Maybe (probably) it's going to suck there, but does it matter for me? It was pretty good on 8MP APS-C (just checked the old review here), and I have no complaints even now. Maybe I have an exceptional copy, but then again, my first copy was also very good and I only got rid of it because I gave it away for free to a good friend in need of a standard lens.

Speaking of native options, well, I guess I would've wanted to go native where possible if switching to mirrorless, especially since some ML lenses are unique and offer advantages over their SLR counterparts (say, this 14-35 vs. a 16-35). Otherwise I might have just as well retained my full EF kit and only used it, adapted, but then what's the point of even switching the bodies?