Maxy
The Gem flies very lightly on the line, very little pull; if you know someone who owns each kite, try them out and se which pulls the least and is most precise. There'll be no fun for your son if one or other is too twitchy and spends more time on the ground than in the air.
That said, I love my Gem and have never found it to be twitchy - it really does manage to be a precision and a trick kite; many kites claim to be this, very few achieve it.
Good luck and happy flying to you both.