I know there is going to be varied opinions on this question. We're being asked to create some mobile maps for various departments in our county. This happens to come at a time when AT&T has bought out our wireless provider and we're all due for new phones. I'm trying to figure out which phone operating system is best for ESRI mobile development. ESRI has the API out for the iOS, and Windows Mobile. And it looks as if Android is on its way. Any opinions on which is better and why would be great.
From the developer perspective I have to say choose the OS and platform that you are used to. If you are a Java developer propose Android, .Net developer use WP7, iOS developer use IPhone. The time it takes to get you started in the platform that you are familiar with should be reason enough continue using the same developer tools. As far as I understand ESRI will try to keep the same level of functionality on all platform even if it's not identical.
I think the prudent action is to forget about the mapping side, and pick the device that fits the organisation/task the best (which is covered by an ESRI api). Once you've got the best fit from a device point of view, then use the ESRI api that goes with it.
IMHO picking he phone based on the api is looking at it backwards.
It is worth bearing in mind, that with Android and Windows Phone, people will be able to choose hardware that closer matches their work function.
ESRI has an API for Windows Phone 7: http://help.arcgis.com/en/arcgismobile/10.0/apis/WindowsPhone/samples/start.htm
It is currently available as a Release Candidate. The code and functionality for the WP7 API is near-identical to what is offered in the Silverlight/WPF API.
That is definitly the way to go if you feel comfortable with XAML and C# programming.
ArcGIS Mobile for Windows Mobile 5 & 6 is still the only API that offers offline/synchronization functionality out-of-the-box for disconnected workflows. Hopefully this will soon be available on the other platforms as well.
Update: So it seems http://forums.arcgis.com/threads/7763-Ad-Hoc-Offline-Use