ADDITION: After all, this is what Khan of Khan Academy does (he uses a wacom though, but then it's a end user decision).
Well, I'd say the central theme is human connection, and then the question is what you need to make that work.
All I can say is that the iPad's screen seems to be over the hump for this! We have beta testers, and they honestly seem to be doing great. You can see a few tweets they've made at https://twitter.com/understudyapp . You might also be interested in this video of it in use - https://www.youtube.com/watch?v=KrdjKvka558 . If you look at it _statically_, it looks bad, but when you see it in motion, you see that it's effectively gesture.
You're trying to visually communicate and gesture, not create an artifact to refer back to. My handwriting on real physical whiteboards is terrible, but they still work.
It would be interesting to try on an MS Surface. To literally answer your question, (1) there are a lot more iPads out there than Surfaces, (2) I worked at Apple for 7.5 years, on the Cocoa Frameworks for most of it, and I'm very productive in this environment.
It makes it very natural to gesture and discuss visually.
Understudy works in proportion to how good of a connection it fosters. You really want the tool to vanish, and to forget that you're not really in the same room. For non-work stuff, like talking to my parents, the phone does decently at that for me, but for trying to collaborate videochat is not very effective, in my experience.
Understudy's whiteboard/videochat does do it for me.
(As far as Android vs iPad – well, have to start somewhere! Also, I worked at Apple for 7 years, so it's definitely the environment where I'm quickest.)