I'm a Career and Business advisor who helps people redefine their careers to maximize their earning potential and fulfillment. Former founder and CEO of Voicekick, Vice President of Consumer Products for Yahoo Search, Director at eBay, designer at Apple Computer and IBM. Ph.D. in Human Factors Psychology.
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).
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.