I would think WM7 would be the dark side - I've used an iphone and found certain things (like notifications) and the constant back to home, really annoying. My old Window Mobile phone was better in that regard, and apple is finally addressing these issues now. Windows Mobile 7 might actually be better than iphone in a few areas, and has a great ecosystem (Office, Zune, and Xbox Live) and multiple hardware options. The iphone seems to be hinging on a perfect new model with each release, and the 5th one (iphone 4) seems tired. Apple's advantage right now is really itunes only, and even that may be a stretch if you are not already committed to itunes.
Personally I will be sticking with the Droid 3, the keyboard makes up for alot - and I hear there is a fix for the radio on the way.