I feel like crying... the second app of this kind for the Ipad, after Camera Lucida. Android also has one, "Artist's eye" and I would love to give it a try! Alas, I have a Windows tablet and there is absolutely nothing even remotely useful in THAT store...
But investing at least another 100€ to get a second tablet? Absolutely not!
I still would be interested in experiences you have with either of the available apps for tracing.