I think the problem with Okafor is if you end up with a guy like Al Jefferson, who is very very good at what he does, but not quite good enough that you can build around it. I would take Towns over him for that reason - Towns has the potential to be a bigger and healthier Al Horford with post-ups, defending the paint, rebounding, shooting, playmaking etc. but even if he is just a solid big who's good but not great he's still a valuable asset.
I think if Okafor doesn't just dominate in the post, he will always be an awkward fit and the defensive deficiencies will be tough to overcome.