I don't know much about Windows Phone 8, but I'd imagine your app choices would be more limited due to there being less developers.
This was my main concern I guess.
I really like the Nokia Lumia, both for its design and the legendary durability of some Nokia models.
I WILL however choose the device based almost solely on the availability of apps as the latter (for me) constitutes the only reason to upgrade.
I find the ability to use a smartphone for tethered shooting (eg. star timelapse from inside my tent & awkward angles) , video monitor (albeit a small one, eg. the "missing" articulating LCD on my 5D) and so on.