I agree, It's much easier to get in trouble with ailerons. While I DO recommend a 4-channel plane to start with, I have NO reservations teaching someone on 3 channels

ESPECIALLY if it is a powered glider like the one above!

Now to answer the original question...

There's really no way for us to tell, but with a wing that long, I would have no problem at least TRYING the 15 before spending money on something bigger.