I will start myself having programmed both for Google Android and Windows Mobile.
In the grand scheme of things, the two platforms are comparable in the amount of joy and pain they cause and it takes roughly the same amount of effort to get things done.
Windows Mobile:
+ C# (popular, easy to learn language)
+ IDE with debugging
- IDE costs money
+ Decent compatibility, debug once - run anywhere
- Compatibility is not perfect - there are two distinct flavors (touchscreen and non-touchscreen) and while its possible to write compatible code it still takes effort
+ Native Emulators for different form factors
- Network connectivity in emulator is a pain to configure
+ Once you configure it you can simulate network conditions very well
-- Windows Forms UI is quite primitive and does not have even basic layout. Huge pain in the neck. I hate it.
- The View and Controller of Model-View-Controller are merged into one (class Form), as a result you can't easily separate View from business logic. Thus it is harder to test.
+ Allocation profiler and unit tests
- No regular profiler, no code coverage
- Stagnant - very little has changed in recent times
+ Large install base (10 mil)
Android:
+ Java - popular, easy to learn language
+ Single UI model for all devices (touchscreen)
+ Free IDE with debugging (Eclipse)
+ Working network connectivity in debugger
- Not as rich connectivity options
- Debugging is half-assed - it hangs all the time (search for "19% bug") and you can't eval random expressions, only local variables. But then again it gets the job done.
+ The Intent / Activity / View model is nice - View and business logic are separated. It's also quite cool that application consists of late-bound activities - basically you can expand existing application by developing a newer implementation of existing activity. In other words, it's hackable that way. The whole way how Google rethought the idea of "application" is very fresh and I like it - it seems to make sense to me.
+ Regular Profiler (don't remember about unit tests and allocation profiler)
+ Rapidly evolving
- zero install base