So who is more powerful between Aatrox and Rhaast?

I think that Aatrox is more powerful simply because he has full control over his body and has more experience in the battlefield, after all it seems that Rhaast was imprisoned in Ionia for years.

I recently saw the Kayn special interactions, and Rhaast states(that when he kills aatrox) "Your skills have not improved, Aatrox!" This implies that he was always the stronger of the two, and that Aatrox can't surpass him.

If they do go with the Four horsemen theme and aatrox is war while rhaast is death, then rhaast.

My guess is neither is stronger, but rather that Aatrox has found a host with less strength of will.