I think you should stick with your boy you have now. He is your first horse, sounds like he's broke to ride. A younger, unbroken horse, regardless of its breed, really isn't suitable for a new horse owner. Even though your boy is older, he still has many long years left in him. Older horses are so much more valuable in the lessons they will teach you.
I know you are working with a trainer, that's really great on your part, but don't get suckered into a horse that will need a lot of work to make him suitable for you. Sometimes having an inexperienced horse on your hands may only cause frustration for you. I can't make the decision for you, but I feel you will be happier you stuck with your good old trust worthy horse you have now. Then when you gain more experience, then you may get a younger horse.