There are a lot of factors. If you do a bit research you will find that..
1) Windows 7.x phones are cheaper but they cannot be upgraded to 8. Android versions can always be upgraded.
2) Windows is forcing all the Windows 8 phones to come with a higher spec, hence you have to pay more; definitely you will get a better product. But you should be ready to invest more. Android has a tremendous market, from low range to big giants.
3) 90% of the Mobile malwares and attacks are following Android, so more vulnerable. No such pain with Windows as of now.
4) Open World. Definitely you will get unlimited power to change the version customized OS or install any software up to any level in Android, but you will stuck with Windows version/apps.
5) What I have used personally Windows 8 phones perform better.
6) Android is always better and moreover, in the coming future, android has more scope than windows!!
7) The customizability on Android is unbeatable and unachievable on other software. From the change of keyboard, browser, email client, lock screen to installing ROMS you prefer, Android is definitely leading in terms of freedom as a user. You feel more like an administrator over a user when using Android.
8) There are more hardware choices for Android. Although Windows is better than iPhone and Blackberry, Windows phone lack hardware choices. HTC 8X, Nokia Lumia series, Samsung Ativ S and one or two more are the only good Windows phones. There are over 20 Android phone manufacturers such as Samsung, HTC, LG, Motorola, Huawei, Sony, etc. Again, you get that choice thing: you want a real keyboard? You want a touchscreen one? You want an expensive one? You want a small one?