It's hard for me to say when you are ready to move to a low wing plane without seeing you fly. But, you're going to be the first judge of when you are ready. The best advice I could make would be to ask your instructor if he will buddy box you on the low wing plane. I know most instructors will be more than happy to help you, I do it all the time for my students. This way you can see how you do on the plane without risking losing it if you're really not ready. Fly a couple of flights and you feel comfortable with it then go ahead and fly it on your own, but if not put it up and fly your trainer for a bit longer. Or another alternative is to fly both of them together. Fly the low wing trainer for awhile and start practicing. Then when your adrenaline is up go fly your high wing trainer so you can "calm down" a bit! Then when you're calm again go fly the low wing....

Ken