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If you plan to write a non HTML5 app then you should really concentrate on Android and iOS only. The other platforms are so small it's not really a problem not having a mobile app. So it "only" requires the double of resources. On the other hand the look and feel will be consistent with the OS and speed increases as well.

I think HTML5 (currently) is only good for thin client apps. Look at the Facebook app: it was so slow and buggy.