Like you, I've played Mario Kart, but I've never played a proper Mario game. Super Mario Odyssey is my first. I've played through the prologue/tutorial bit and I'm really enjoying it. I remember trying the one for Nintendo 64 and not liking it because I just couldn't wrap my head around the controls and freeform movement. This time around I'm doing much better.
Since you're uncertain, though, I suggest that you buy the physical version so that you can easily return or sell it if you don't like it, without taking a chance with Nintendo customer support trying to get your money back on the digital version.
Actually, an even better option: a lot of libraries have Nintendo Switch games now. See if your local library (or a friend) has it so that you can give it a try for a little bit and see if you like it before you make a decision.