I would go with the younger just because your a novice and the other horse hasnt been gelded long. He might be great with the current owner but they know how he is if he has had any issues or getting excited... plus witha younger horse you could grown up with it! Rockys are gorgeous and amazing breeds!
The old rule that the "younger the rider the older the horse" still holds true in virtually all circumstances.
Put another way, "green on green equals black on blue."
If neither horse is suitable (and that may well be the case) then the OP should cast their net wider. There's lots of RMHs in the world.