Odd, because the consensus from what I can tell is that Fox is already a capable defender that could develop into a really good one. At UK, he gambled a lot but so do many of the top level players in college.
Even if you don't believe he is already a capable defender, he has proven to be a lot better than Smith on that end of the floor. So they're not comparable in that aspect at all, IMO.
With regard to offense, UK was so deep and talented that I don't think Fox was able to showcase everything he can do. While we all know he's got work to do on his outside shot, I firmly believe he's a far more capable playmaker than we've seen. I'm also not convinced Smith is better on the offensive end other than having the better J right now. Fox is only 19 and will fill out over the next couple seasons so I'm not worried about his thin frame.
With regards to defense, De'Aaron reminds me of Wooden's saying, "Don't mistake activity for achievement"
Fox is very ACTIVE on defense. He's not all that effective on defense though. At least not in terms of man defense. He doesn't properly slide laterally and gets beat a LOT. He then tries to use his quickness to recover. Westbrook does the same thing. He's flying around but he'd be better off taking better angles and he'd get better results with less effort.
Can he be a good defender? Absolutely. But he has to clean up his footwork and do a better keeping his hands up and in position. Fox is a pest off the ball and pretty good defensively in that regard.
Smith can pressure the ball well when he's engaged but his lack of height and length is going to hamper him on the next level, especially if he doesn't give max effort.
Fox is actually a better midrange shooter than Smith, but significantly less effective from three and finishing at the basket. Part of that is his frame too. He doesn't shy from contact, but it does often prevent him from finishing. Smith finishes better through contact.
I think getting stronger will open up Fox's game but it concerns me a bit that he doesn't look like he's added much (if any) muscle since high school. His legs especially still look super thin. He may be a hard gainer in terms of muscle/strength.