Switch. That's where the kart fans are.
But, i expect a close race with the ps4.
Given how transformed sold on the Wii U, it's a reasonable expection. Though, we must remember that the Wii U version was the superior one at the time (and there was no MK8 yet i believe), but given that the Wii U had a smaller userbase, it's a reasonable expectation.
We'll see.