I think it depends on the kind of startup. If your startup is trying to grow an audience around a service with a free app, Android is a good place to start, if you are dependent on sales of the app for revenue, I'd go with iOS.
The app was never intended to be paid so making a free app with the ability to use a proprietary in-app payment mechanism (PayPal) was what lead to a really easy decision (PayPals Android Developer Challenge was another nudge into the Android direction).
The next step will be a webapp to which i'll redirect the iPhone customers. I didn't start with it initially because from the little research i've done, "wizard of oz"-style, people didn't find iPhone webapps stable and trustworthy enough to Donate Money via them.
They tried to make a big deal out of it, trying to show that it could be a downward trend, but it's nonsense. Android sales keep growing worldwide, and I'm sure it will rise above 50% in US by the time iPhone 5 launches (and take a hit again for a few months, and then rise again).
Android has 38% install base vs 27% for iPhone.