There is not a lot between them power wise but I think Stevensons delivery is better, he can close the gap between him and his opponent really fast and his straight left is often coming from weird angles where the opponent doesn't see it coming. Kovalev seems to have a better chin and more variety in his attack while Stevenson has better athleticism and footwork, I really don't know who I would pick if these two were to meet.
I'm pretty sure most anglo sax's will select Kov and most sub-saharan's will select Adonis.....truth is we have no idea and no scientific evidence proving either or. What we do know is its best to avoid being hit by both guys.