I'm deep into the google ecosystem, and I especially like the seemless integration between mobile and desktop (for eg. Gmail, Keep, Hangouts). An experience MS cannot yet replicate.
And while WP8 interface is very polished, I prefer the simplicity of Kit Kat on my Nexus 5. I set up a Lumia 520 for a friend. Initially I was impressed with UI, but after some use it's lack of customisation options for so many settings was frustrating.
And then there is the very large elephant in the room. Lack of apps. So many apps I enjoy and rely on are not there ... and unlikely to arrive in the immediate future.