My company has prototyped on both platforms, and I strongly prefer Android for this purpose. Test flight is significantly more inconvenient for all parties than sending around Android apks. So is Apple's provisioning process for development (you'll certainly have to figure out how to re-provision your development devices a few times).
Android's OS integration also makes for better demos of our app. Since it's a remote control for another device it makes a lot of sense to have lock-screen and pull-down controls. We can't show that off on iOS.
Finally, the Android developers that I know simply don't use the simulator, they always use hardware. I found it strange coming from iOS, but I've gotten used to it. I weight this negative much lower than the deployment advantages that Android has for prototyping.
Of course in the end you'll have apps on both platforms, but here's a vote for doing early development on Android.