I think he's going #1 overall. It would be one thing if Okafor was significantly bigger and more athletic, but he's not. The only thing Okafor has right now is a more developed post game (and Towns has made huge, huge strides in that area), and strength (and it's not like Towns is small, he just doesn't have as mature a body as Okafor- something that could change with age/training).
I saw someone compare Okafor to a great running back in football- something that can be incredibly useful, but not necessarily something that the best teams in the league actually need, due to changes in how the game is played. Towns does so many things better than Okafor that I think the majority of teams aren't going to be able to pass it up. And that's not even counting Towns' spot-up shooting ability, something UK hasn't utilized, but something a lot of NBA teams are going to drool over. Towns is going to do some workouts for NBA teams and just nail jumpshots, and that will cement him at #1 overall.
I was a little skeptical about Towns, as I doubted whether he had the explosive athletic ability to be a star. His work ethic and attitude have blown away all doubts.