I got the RF 14-35 f/4L IS USM lens and love it. It is much lighter than the 15-35 f/2.8L, more compact and cheaper, and IMHO there is not much between the optics, and at the wide end, 1mm can make a difference. Also, the 14-35 lens will take a conventional 77mm filter without an adapter, which is great if you want it for specific effects like ND or CPL.
This is a in-depth review by Justin Abbott, made before the availability in RAW corrections for Adobe, so keep that in mind.
The lens apparently shoots at about 11mm, with significant vignetting and some distortion, and uses that 3mm with computational algorithms to clean everything up so that lines are straight and no vignetting that I have observed, even with a CPL. This is automatically done in-camera with JPGs and also done as a part of lens corrections that are applied automatically as RAW files are brought into Adobe software - why anyone would want to turn this off is beyond me. Many of the earlier reviews did not have access to the Adobe corrections, so at the time they were critical of this, but since the corrections have been made available, I see no issue and expect this correction technology to be applied in more lenses in the future - it is already for the RF 24-240 and, of course, has been used extensively in cell phones for a long time.
These were shot in RAW, brought into PS with auto correction applied, and reduction in file size to fit this site.
EOS R6, RF14-35L@35mm, f/20, 15sec, ISO-100 
R6, RF 14-35@22mm, f/18, 15sec, ISO-100
