I do it this way, but it's based on my particular skill set:

First make the iOS version. Then, port it over to Java. Then, port it over to C# or maybe ActionScript3/Flash.

This way, I can recursively update previous versions as the 'best solution' to interesting problems become most clear by the end of the 2nd or 3rd port. This gives the Objective-C/iOS version the attention it needs, and I can use the rapid application development features for each new port.