As always, trying to say one player is "the best" is an exercise in futility. They don't have the same supporting cast, didn't play the same teams. Mahomes has Andy Reid for a coach, a long term, proven offensive/quarterback developer. Dak had Wade Wilson and Linehan, now has a rookie OC in Moore, so for him to play as he has could indicate he's more talented than Mahomes, to do as well as he is doing without a Reid on the sidelines.
And you have to factor in Mahomes is in a bit more pass-oriented offense, Dallas still is more run-oriented, just look at the number of passes attempted - Mahomes 77, Dak 62. Rushing attempts - Dallas 64 KC 48.
Finally, it's only 2 games. Teams are relatively healthy this early, but now with Gallup out, we should have a bit more indication of whether or not Dak's the "real deal". Of course Hill didn't practice this week, he may be out also, so that should give an indication on Mahomes as well.
Just way too early to really judge who's the best...