It's very close, of course. Let me do the "math". I know that tennis is not the sum of parts, as there are many intangibles as well. But roughly speaking, Novak and Rafa are of the same caliber, so let's look at everything outside of their groundies.
Novak merits the edge on both serve and ROS.
At net, including overhead, I'll go with Rafa.
Court coverage is roughly equal. Rafa slightly more sheer speed, Novak with a little more length and elasticity.
Both are incredibly mentally tough, with similar intangibles.
So, maybe the slightest ovrtall baseline edge to Rafa, as he had roughly the same success with a lesser serve and ROS.