For really rapid prototypes than can be simulated on an actual device, Keynotopia is a great tool. The UI look can be identical to the platform OS. The interaction is tap-based, so you won't be able to truly simulate other gestures like swipes, pinch and zoom, etc (unfortunately). http://keynotopia.com/ With regards to your development question, cross-platform hybrid app development is very tempting (e.g., Phonegap), but I would not recommend it. My own startup went down this path and I deeply regret that decision. Sooner or later you will want the deeper integration and performance of a native app, which hybrid apps cannot achieve, despite their promises and protestations that they can. We eventually had to abandon our hybrid app and build a fully native one. Many others tell the same tale.

Lyft was founded in 2012. They operate in 200+ cities in the U.S. cities. They've raised more than $2B and they're valued at $5.5B. Udacity was founded in 2011 and is valued at over $1B. Vox Media was also founded in 2011 and is also valued at over $1B. Uptake was founded in 2014 and is valued at $1.1B. Oscar was founded in 2013 and is valued at $1.7B.