Based on what? Kei is clearly the more talented player of two, serving is basically the only thing Milos does better. And while Kei will never serve like Milos or even remotely, his serve does have margin to improve.
Serving could be key especially for an attacking player. You wonder if Fed would have continued to stay at the top of the game with these super-fit grinders if he couldn't serve his way to cheap points.
Raonic has enough ground game and talent to win return games. I like Kei but not sure if he can win cheap points on serve and also he can be erratic sometimes still. I hope they both get to the top though.