I chose PHP. I tend to do whatever I can to do NOT be dependent of Microsoft as there's always a $ sign somewhere. Other than that, PHP is everywhere, it's free, it's fast, it's powerful, it's easy.
After using both, I would say ASP.NET. ASP.NET is a much more elegant solution, in my opinion. I would just try both out for a bit to see what style I was more comfortable with. Good luck!