Here's the long and short of it.
I like RJ Hunter as a player. He's heady, shows basketball awareness and tries hard. You can tell he's a coaches' kid with a decent basketball IQ. He's easy to root for. But he simply doesn't have the athleticism to play in the NBA. His lateral quickness is average, he can't create his shot, he's too skinny and weak to not get pushed out of position and his team defensive skills are mediocre. If he could have proved that he's a sharpshooter then he may have had a role off the bench but it didn't fall in his last year in college, last year or this preseason. It looks good coming out of his hands but it doesn't fall enough. If he could have transformed his body this offseason so he's not getting bounced around on the court then perhaps Ainge could have waited on him. But he's 23 and neither his body nor his shot have impressed or improved. There's simply no more time to wait on him given his age. It could be - in fact, it's likely - that RJ Hunter has peaked as a player and will never get any better. I don't think it has anything to do with effort. It just may be that RJ Hunter is as good as he can be but it's just not good enough.
Young, on the other hand, worked harder this offseason than he ever has before and it showed. His body looks better. While his BBIQ still isn't great you could see at least some improvement in his game. Even though Young is not Jaylen Brown, he's still physically more gifted than Hunter. He's almost 2 full years younger than Hunter. Young showed progress, determination and perhaps has finally learned how to be a pro. It's not that expect either of these guys to stick in the league very long but it's hard not to argue that Young is still the better prospect than Hunter. It's harder to root for Young because he's been lazy for two years while you can tell Hunter has busted his butt. But a motivated James Young is simply a better prospect than a motivated RJ Hunter.
Contracts notwithstanding any "tie" between these two guys goes to Young. Hunter didn't outplay Young this year. We don't know what happened in practice and Hunter didn't prove it on the court during the preseason. So the big issue in keeping Young over Hunter was his contract. It's a bit more and has one fewer year. Ainge valued potential over these contract considerations and that's probably the correct decision when you're selecting your 15th man.