Personally, I am building an app for the iPhone first because of public perception and marketing. It seems more people write and review iPhone apps versus Android apps - you can just tell when they start Android review articles with a tone of appeasement ie. "Ok, finally here's something for all you Android users.."

When someone finds your app and sees there's an iPhone version but no Android version, their normal reaction is: "That's understandable, Android is coming next, right?". If you have an Android version but no iPhone, their reaction will probably be "Hmm.. wait, that makes little sense...". It's just a side effect of iPhone being the first one to come up with app store.