For prototyping, I think the best tools facilitate interaction and describe functionality without being overly high fidelity or requiring any design skill. (This is particularly important for me!)

I find sketch interesting, and mirror is very cool, but it is more of a tool for designers, IM (very limited) HO. Similarly, PhoneGap and Appcelerator are for something else: actually building something. The code-ish nature of those products is the exact opposite of building a lightweight prototype.

I have always used Omnigraffle to build prototypes both for mobile and the web. I find that it is easy to print to HTML, dump it on the web site, and have a similar clickable prototype to Mirror with very little work. Omnigraffle is not super cheap, but it provides the flexibility of allowing you to go very deep in the prototype if you want, layers that make building functionality quick and easy, and simple, high fidelity UI components.

Happy to talk more if you want to set up a call.