WP7 for me, well at least for the last four days...here is why
GPS...Instant no waiting at all for a fix, using Endomondo for running, the route is spot on, best i have ever seen on any of my other phones, or even this HD2 running WM to Android.
SOUND...the sound quality, through headphones on WP7 is second none, why on eaarth its so much better than Android I really don't know and the ZUNE player and online stuff is simply fantastic
OFFICE...miles better than WM office, Android office apps just don't cut it
GAMES...try Harvest wow can't believe my HD2 is actually running this. loads more to try, also loving AlphaJAX bit like scrabble and wordsmith on Android only slicker.
INTERFACE...slick super fast, makes Android and WM on the HD2 seem antiquated
SOCIAL...the people update is simply perfect, facebook and twitter apps never look so good, loving the swiping about
KEYBOARD...never used such a easy to use keyboard, not sure why its so much better, I can type faster then on swipe, weird
Camera and Video... well this is hands down miles better than anything else for the HD2, did some filming at the weekend and the clarity is amazing
CALENDAR...quick easy and perfect for me to add and adjust my busy schedule
PHONE and SMS...sound quality and signal just work
Ok there are few teething problems but hopefully between XDA and Microsoft these will improve, ie...cut/paste, flash would be nice, but uses to much of my 3g allowance, Multitasking not that it needs it, it runs so fast, but perhaps being able to run my online radio with the brilliant rescoradio and do other things would be nice, otherwise you can have music running all the time and do other things, lets see what happens...