If you're used to writing Microsoft compatible software, WP7 will be easier, iPhone's the same for OSX, Android's probably a bit easier if you're happier in Lin

If you're used to writing Microsoft compatible software, WP7 will be easier, iPhone's the same for OSX, Android's probably a bit easier if you're happier in Linux land but I wouldn't know. WP7 right now has a small market but isn't flooded with products. You have an opportunity with Windows Phone 7 to gain first mover advantage with your product that you won't necessarily have with Android or iPhone. With iPhone, charging for your product, unless it meets a particular niche is unlikely to be viable in the long term. If you can make money from in-product purchases and can promote your product in a crowded market, iPhone is probably the way to go right now. Android is gaining popularity and the question with Android isn't just phones but what else would you run the app on? (e.g. TVs) Incidentally you have the same question with iPhone (iPad, iPod Touch and a possible future AppleTV store). I would recommend that you look at the app you want to make. If you pick something fairly simple that exists in a niche without too much competition you can test the water for each. You might need to rewrite code, but you would be able to reuse algorithms and patterns in a large amount of cases.