Mechanically I would put them at a similar level, and I feel both games can learn from each other, I favor stuff like the drift and boost usage from Crash, while MK has a better feel in its control and general race tracks design.
They are on par in terms of content I think, Crash has the cool story mode, while Mario Kart I feel motivated to go through the Grand Prix Cups.
In presentation Crash can't touch Mario tho, neither music score, art direction or character design, that gives the edge overall to Mario Kart.
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