I've been switching them in and out playing next to tots varane, and even tried them together.
For me Upamecano is much better. They're similarly fast, but Upamecano is so beefy he just throws attackers off the ball. If anyone has varane and is considering one of the two, I'd go with Upamecano any time
It pains me to put militao on the bench, after such and expensive SBC. But Upamecano wins me more games