While I don't totally disagree with you, I think it's a hard call one way or the other. Personally, I think Giles had better handles and a better jumpshot. But as they say, beauty is in the eye of the beholder.
Giles had better handles for sure and for sure was more confident in his shot.. He was able to handle the ball on the break at 6'10 at that age in spectacular fashion, crossovers, spin moves, nice dump-off passes. Harry had a dazzling array of tricks up his sleeve for a big that age. His footwork while approaching more calculated.
Marvin to me tho is different, he is just more of a natural, it's like a timing thing... Plus you know I like to try and gauge "Staying Power" as Harry's stock deteriorated week-by-week I couldn't help but think of Marvin and conclude "This is not gonna happen to him, he's going to be drafted right at the top"
I'll always remember seeing a young Andre Drummond like 9th or 10th grade and thinking only some crazy off-court tragedy can stop this kid from making the NBA, I kinda had the same feeling seeing a young Harry and Marvin.
FWIW --- if we're gauging shooting at that stage Skal was ahead of both of them.... I believe he shot 88% from the FT stripe and took all the technical attempts for his HS team Sophomore year (back when he was playing in Tennessee and Memphis was perceived as his likely college destination)