Having had an Android phone for a year, and now I've had a WP7 device for the last 3 months
WP7 is consistently fast and very smooth. I really like it's music player (It's replaced my iPod now). Hands down beats android for music and speed (except the Galaxy S2, that's the only android phone I've seen which is as fast as WP7).
There aren't anywhere near as many applications for WP7 as there are for Android, and there aren't that many free games etc. If you absolutely must have loads of free applications then WP7 isn't ideal.
Basically;
WP7 is faster, easier to use and has a better Music Player than android (including third party Music apps on the Android Market)
Android has more applications in general and definitely more free applications than WP7.
To be honest even though they aren't as many free apps there's a giant load of apps that have very good demos which work fine and show no adds. So in some ways I prefer good demos, no adds and buyable full versions to everything being riddled with adds yet free.