[credo]

Since you mention that market size isn't a factor for you and since you don't seem to have a preference for one phone (though you've tried both), I'd suggest Android.

I wrote a prototype app with the Android beta last year, The Android beta platform was fairly rich in functionality and had a lot of stuff (background process, advanced location-based functionality etc.) that iPhone 2.0 did not have. Java programming on the Android will be closer to the (managed code) C# experience you currently have. So your overall rampup time with Android will be lesser.

Btw I myself chose to focus on the iPhone because I liked the device (as a user) and because there is a much bigger market for iPhone apps.