Hello,
Good choice! I have an Avistar which was my second plane as well. It is a great flying aircraft. Mine is now on floats and still flies great. I would put the .51 on it right away, the 40 LA was underpowered in my opinion. I did pretty much anything considered basic aerobatics. The plane wasn't really designed for the 3D category. Recently I was flying and able to do 5-6 loops before I had to add any inputs to straighten out the plane, and thats with floats underneath. As I recall it snapped well and also did pretty nice spins and slow rolls. Go easy with the snaps and make sure you put a couple extra rubber bands on before you try one. You should really enjoy the plane.
As for a 3D plane. The skys the limit there. I have a fellow club member who is flying the Great Planes U-Can-Do .46 and has had great results with it. It lands like a tranier and still does anything he asks of it. My Ultra Stick 40 is also a good choice. Be sure to put plenty of power on it and it will treat you well. I'm sure you will have many more suggestions. Jeff