Oooh, that's a toughie.
I'm going with Young. In 332 minutes last year Young put up 66 3 pointers. His make percentage was abysmal, but he's too good of a shooter for that to continue to happen. Interestingly enough, Amir's max in a season is also 66 (in 2,200 minutes). I know he's more likely to put them up at a greater rate in Brad's system, but I don't think it will be substantial. I expect him to get paired with KO, who will be Mr. Outside to Amir's Mr. Inside.
I kind of figure that Amir will only play about 2,000 minutes while Young's playing time may increase to 500 or so minutes. I think Young will play some this year. If that'st he case, Young will fire up more attempts and probably more through the hoop.