A year ago I made the hard decision between Android & WP7 and eventually went with WP7 for 3 simple reasons:
1) I love the UI. Clean, simple, to the point.
2) Responsiveness of UI. Every Android phone I'd played with (a year ago) didn't feel great. It felt slow, and clunky - almost as if smartphone CPU speed couldn't handle all that Android. WP7 felt fast, and I'm an impatient guy.
3) Effortless Social integration. BlackBerry had none. iPhone - I felt I needed to spend too much time looking for apps to get all the features I wanted. Ex: I had to download 3 different apps to: a) post quick facebook updates, b) sync fb contacts to my address book, c) view my friend's facebook updates. As soon as I set up my WP7, all my Google Contacts and FB contacts synced automatically, linked common data together, and I could see their FB updates all in one spot! I did nothing, and I had John Smith on Gmail & FB linked with email, phone, birthday, fb wall, and so on. Little things like this make me smile.
BlackBerry did my business well, but lacked quality apps and wasn't very fun.
iPhone did my business well enough, and was entertaining - but managing all those apps felt like a chore.
WP7 already did 90% the things I wanted to do, so I only needed a few more apps to handle minor things.
As Elop said - it's a battle of ecosystems, and with great dev tools & the XBOX Live brand, I took a bet that good apps/games would eventually come. So far - I'm still happy (probably because I'm running Mango).