Where did you get those numbers? The league is shooting .427 from 10-16, .401 from 16-3p and .349 from 3.If there could only be one, and If you could make the call, would you rather see if you could get RW3 to begin developing a 3pt shot? Or would you push him to develop a 15 footer From baseline to baseline?
Keep in mind that a 3pt% next year might be well below 30% while a 15 footer might be @ 42%.
(of course the point would be for steady improvement until one was a consistent weapon. but NEXT YEAR those are your options)
So you think he can develop a league average 15 foot jumper as soon as next season? To answer your question, I'd rather he develop a 3 point shot. 28% from 3 is equal to 42% from 15, and assuming he can shoot 42% from 15 feet is being extremely generous. 28% from 3 might be pretty generous too but it seems more plausible for a player learning a new skill to do it at well below league average than at league average. Plus the sooner he starts developing the 3 point shot the quicker he develops it. That and I don't see much point in developing a 15 foot shot in today's game unless it's a form thing/mental thing/confidence thing. Just skip the midrange and start from 3. If he is absolutely dreadful from 3 and it looks like a lost cause, you can always go back to the 15 footer.
The game is mathematically broken.