Neither. In my view it is a bad time to being buying a phone. While a few years ago Apple owned the market, you are now seeing all of the phone manufacturers trying to carve out a little niche in the market for themselves. Until they find what it is they
want to be doing you might very well find yourself with an unsupported product in two years time (24 month contract).
Android still needs work and a lot more community support. Windows 7 is vaporware at this stage and Apple is locking down the iPhone so hard it is hardly fun any longer.
One thing about WP7 that's not vaporware though, the awesome dev tools. Screw Android's Java implementation or writing objective C on a Mac.