I think Kane is better and there is more he can do. To be frank I haven't seen enough of Lukaku since he left us to believe that he has improved significantly since then.
I get that logic 100%, but part of me wonders if Kane won't slow down City's build-up play even more. He likes to start his runs from deep and arrive in the box relatively late for Spurs, and to get on the ball around the half way line. That's the same as basically all of City's forward players. Everyone talks about Kane like he's a model box striker, but I haven't seen him do that for a while, even for England when that was what he was meant to do.
Do Man City really need a false 9? I always got the sense that for Spurs Kane guaranteed goals at the expense of team fluidity, and whenever he didn't start the team actually seemed more cohesive. I think every Spurs player deferred to Kane in every situation, and there's still a bit of a question about what happens when he isn't indisputably the main man.