For various reasons, iOS users are empirically more willing to pay for apps and services. There are more Android users than iOS users, but iOS apps still generate the lion's share of mobile revenue.
Some sources for this (based on a quick Google search, one could probably find better or more conclusive sources with a little legwork):
2) What you can make hugely varies. $0-$500k+/day for serious games. It is very skewed towards top earners.
3) You'll probably have an easier time selling and developing on iOS.