If I'm the Spurs and deciding been Castle and Harper (two prospects I personally really like, btw), I think I would keep Harper since I think he is a better prospect overall. You might also take advantage of Castle winning ROY, and potentially having a high trade market.
On the other hand they might like what Castle does better, and be more inclined to stick with what you have, and I'm sure that as high as Castle's trade market is, they can get more for Harper.
On the other hand they might like what Castle does better, and be more inclined to stick with what you have, and I'm sure that as high as Castle's trade market is, they can get more for Harper.