Unix, PHP, MySQL, in my opinion, are the best combination you can ever get...
Besides the fact that php and mysql are free software, the documentation and forums about them are really good...
I´ve worked with asp for a short while and just couldn´t get it right... There was not a good documetation, was rellay slow to develop...
And besides, any OS is faster then win NT as a server plataform, especially with IIS...
Dude, stick to the Free software.. you won´t regret