The "problem" is that Harper is by far the second best prospect in the draft according to pretty much everyone. Not taking him would be a disservice to the organization (at the minimum as an asset).
Harper and Castle have some similar qualities but I think Harper (at least) would benefit immensely playing next to Castle, or Fox. He was a much better shooter off the catch, rather than off the dribble (IIRC).
The plus for Castle is that he is a great secondary playmaker and doesn't necessarily need to be the PG, and he is already one of the best perimeter defenders in the NBA without being a complete zero on offense. Even if one of the two comes off the bench while Fox is there, there is a ton of minutes to get them all thirty plus, but the coach will have to be on point with rotations.
I think the Spurs are hoping they end up with their Cade AND their SGA in about 2-4 years.