It may not be fair to compare an obsolete OS with a strong growing, main stream popular OS.
Purely on Sat-nav device point of view, I prefer Android devices for:
1. For hand-held units, the battery life of android units is mostly much longer than the WinCE units. This may be the single most important factor for some.
2. You can choose screen size ranging from 3.5in to 12in, with many resolutions. If graphics/display is important to you, there is no choice but android units.
3. Flexibility: android has greater flexibility than WinCE and IOS.
4. Costs for android units drop faster than WinCE and IOS. Now I put a generic android phone (no SIM card in it) in my car as my Satnav device as it costs less than AUD150. It gives me a backup for my phone in case my mobile goes down or runs out of battery.
5. Apart from Garmin, all major players in satnav are available for android devices. Sygic is better in Android than in WinCE.
6. Once setting up the programs and running, android is probably more reliable/stable than WinCE, at least there's no errors like iGO and TomTom for the WinCE machines.
7. Added advantage of having many other software. You may decide to use your device for something else if you want another gps for your car.