I think CRT have more difficult maps with better elaborated shortcuts. You need more knowledge and ability to master CRT tracks than to master MK tracks
About art direction, I also feel CRT have more elaborated scenarios and more detailed character design. The CRT maps are gorgeous with more things happening in the background, more vibrant colors and more complex textures and assets
One thing that bugs me a bit are the loading times, they look bigger in CRT compared to MK
But I really miss a story mode for MK8, I understand most use it for the multiplayer, but I like singleplayer and I beat all maps in 2 days and then felt unmotivated to play more
CRT story mode is harder, has time trials and CRT challenges, and since it takes more time to master the sinolayer playthrough is lengthier
From my experience, the better shortcuts are more in exceptions rather than being the norm with Crash, there being more to master in Crash comes down to the boost and drift rather than the tracks themselves being complex I feel.
The thing with art direction for me is, by default Crash may have more detailed visuals just based on the fact that it was made with a more powerful hardware from the get go but that doesn't translate to a more aesthetically pleasing or better looking game to me, there is more detail to the character models, but Crash's designs are very generic compared to anything from Mario, race tracks may have more going on, but nothing is as inspired or there isn't as much thematic variety as the race tracks from Mario, plus there are some race tracks where the palette clashes with one another in Crash, leading to a not very pleasing contrast.