I prefer PHP just out of circumstance. The company I work with has over 50 web servers and yes about 5 of them are windows boxes, the windows boxes are not used for front end applications but instead for windows type programs like monitoring tools.
I prefer php its very much easy to learn and code, follows the basic syntax of C language, is a server side script, lots and lots of functions to work with, and a friendly community to help you along
Although I learnt both the PHP and ASP, but I would rather prefer the PHP, especially for those amateur that want to get involve themselves in the computer scripting languages.
The PHP is a lot more easy to learn and the best thing of PHP is absolutely free!