nope, both can run the exact same ultra textures in rdr 2. there are no additional textures that rx 400 can ran on top of gtx 970 in rdr 2, ac odyssey, god of war and many more lastgen titles. with optimized settings, there's enough vram headroom that alleviates any potential stutter problem
if you push high fidelity settings that push higher vram usage (shadows, reflections and so on), performance drops so much on rx 480 and 970, both get unplayable framerates, so they have to use selective optimized settings either way.
here's actual proof
rdr 2, at 1080p, with one x settings, use only 3.5 GB vram out of a total 8 gb vram available by rx 580 which is what rx 580 and gtx 970/1060 targets
and hey, look at 1080p ultra settings,
now we are talking. a whopping 4.7 gb usage, innit? that would surely show gtx 970 its day. oh wait a second. 30-35 fps. not good !
this is what being forced into your natural memory usage limits look like.
you can either use optimized settings and get a pristine 60+ frames with adequate vram usage (3-3.5 GB, like consoles), or you can push and use "unused vram" with high fidelity settings and enjoy 30-35 frames.