You probably read my post about how much I love ScreenFlow! That is my workhorse for the Mac. For my iPad, I am equally as stoked to use ExplainEverything.
Simply start recording, talk and doodle, and everything you say and draw will be recorded. When you are done, there is an automatic upload tool so you can send this straight to YouTube.
In my disciplines, there's a lot of evaluating, considering and rough notes involved. When I map gene locations for my students, I have to create a symbol system, do sample calculations, and sketch the map. I find that this is the best tool around for modeling the discipline. It is amazingly fast: if I want to do a five-minute production, it takes me five minutes. Uploading takes a little longer, but I just set the iPad side, and let it do its job.
You can get this from Morris Cooke. (for under $10, I think!)