Both three years younger and within about 100 rating points.
Isn't Nakamura ahead in blitz?
I bet So.
Some facts in classic games:
Carlsen against So -> 4 to 1
Carlsen against Giri -> 4 to 1
Carlsen against MVL -> 7 to 2
Carlsen against Aronian -> 16 to 7
Carlsen against Caruana -> 10 to 5
I don't think either but Giri if i had to choose. Carlson gets bored and likes exciting chess Giri would probably just play the Petroff and try to bore Magnus to death. No offense to Petroff Players lol