This is strictly a general bit of advice. The shape/spread/angle of the tree trunks will have a bearing on which size rocket will work best.
If I had to flip a coin, I would say that the larger size will give the user a little more needed space in the "average" tree (whatever "average" means).
In a tree with more parallel trunks, I'm thinking that the larger one would be better.
In a tree with trunk spread on a wider angle, the smaller one should work okay, too.
Either size will dictate where in the crotch it will work.
I tried one...it was rock solid.