I assume you were using the 20 APS-C lens on a Canon body.
If so, I would indeed go for the RF 35 in that case. I happen to own one, and funnily enough it is the first ever 35 mm FL lens I actually like shooting with, although I would normally prefer a 28 or 24 mm. And I can assure you I have tried many ....
I see a 35 mm FF lens as a short standard lens, and it is in principle just that little wider for slightly spacier feeling shots in tighter spots, without really having the perspective feel of a real WA lens.
The advantages of the RF 35 over (most) EF lenses are that it has good IS, is light, and has "macro" as well, up to 1:2, which is great for the occasional (handheld) close-up shot. It is also amazingly good, almost in EF 35L territory.
I happen to own the EF 40 F/2.8 which I used a lot as well in the past for the same purpose, but it is of course F/2.8 rather than F/1.8, has neither IS, nor semi-macro facility, and is only about 9 or 10 mm shorter with adapter on an RF body. It is also not as sharp.
HTH, kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....