While I assume Bill Self's reasoning behind starting Frank Mason over Naadir Tharpe is supposed to be a wakeup call for Tharpe or Self's way of reassuring Mason, who had a terrible end of the game against UTEP, that he still had faith in him, this is a move that will hurt Kansas in the long run more than it will help it.
Now, in graph form:
I'll start with the defense. Mason has the edge in steal rate by about a percent, but he's been beaten off the dribble by his man a lot more than Tharpe has, and he's been a worse team defender as well. Tharpe won't ever be Wiggins or Releford defensively, but he can adequately guard point guards, or shift over to guard a weaker 2 when Selden or Wiggins will have to shift over to take a tough point guard.
Mason's big edge, obviously, comes in the form of a lower turnover rate. While it's true he has generally taken care of the ball better in terms of turnovers, I think we are dealing with a bit of fools gold in that case. Mason takes almost a fourth of the shots available while he is on the floor, whereas Tharpe is at just 14.6%. With Mason shooting what he shoots, those extra shots function almost as turnovers. Mason's best shot is actually a 2 point jumper, as he's making 60% of them, with him shooting under 50% at the rim and making just 2 of his 14 threes. While it's true that him driving the lane has caused him to get fouled a lot, it also causes a lot of misses and, whatever your feelings on how good Frank Mason is or can be, him driving to the hoop is nowhere near Kansas's best attack offensively.
Tharpe, meanwhile, has a better understanding that his job is to be a facilitator. Last year he took 20.6% of the shots while on the floor and I hypothesized that number would go down with the increase in offensive talent and guys who could create their own shot around him. And it has. Tharpe has shot less this year, and he is shooting 35% from three, making him a better outlet for KU's post players than Mason. I am willing to bet once the team gets a feel for one another Tharpe's turnover rate will drop to around 20%, and as long as his assist rate holds steady that will be plenty good enough for a starting point guard of a Final Four caliber team.