ASP Vs PHP, Who wins?

I've used php before and it's a great language to learn and use, i've also worked with asp (briefly on a project of mine). There seems to be a lack of qualified asp programmers out there and a definite shortage of windows hosts when compared with unix.

This got me thinking, why is this? ASP is Microsofts language, is it not? I was just wondering why isn't ASP as popular as php, nearly everyone I speak to is either learning or codes in php/mysql but hasn't even tried ASP, why isn't it the language to learn? Are there any major benefits of PHP over ASP or is it just a personal preference?

I know alot of the big companies/industry use ASP but I can't understand why, PHP is a great language and has alot of flexibility. Maybe someone can put me straight on a few of these questions!