In the finale of the storylines both Rimuru and Naofumi become omnipotent deities that can control all existence and reality and this happens often to Isekai main characters so this is a difficult choice, i guess it would be stalemate unless Rimuru finds a way to bypass the defence of Naofumi.
@devoidruby: don't know about the anime version but in manga version rimuru stomps naofumi
As for the novel, naofumi was able to tank conceptual attacks from medea a goddess who can easily destroy universes with timelines like past,present and the future and he can reflect such attacks back at his opponents.
heres what gets me about this, theyre both pretty even in my opinion, until rimuru pulls out the hax, multiple colnes of him at the same power level just might be too much. i would perfer them to be friends than enimies.