My opinion would be to first keep flying your Avistar until you are extremely comfortable with it. Then you could move on to the GP Mustang if you had a very experienced pilot fly it with you on a buddy box. That way you can gradually get used to it's characteristics. I have seen so many just solo'ed pilots jump into a hot second plane and then smack it into the ground because it was too big of a step up. I have heard that the Mustang flys very well for a semi-scale plane. Just ease into it, and good luck!