Currently Online 
It will depend on the games as usual. Theoretically being able to load data twice as fast won't make much of a difference when additional procedural generation by the CPU is the bottleneck. Not every tree and rock are stored on disk anymore, most are generated on demand just like NMS and Minecraft. Resolution and frame rate also depend on the type of game.
PS5 will have the edge for install, loading times and less noticeable lod changes, XSX will have the edge in stable fps and resolution. The differences in the latter will indeed be less than between XBox One and PS4 in the early years and much less noticeable especially with VRR. The difference in loading will be highly variable like it is now when you put an SSD in current gen consoles. (which are severely bottle necked by the IO bus, but still offer up to half the load times for some games in some specific instances) However on both consoles those times will not be comparing 77 seconds to 44 seconds, more like 3 seconds vs 1.5 seconds in the best possible scenario. And games with heavy procedural generation will vary much less.
