PHP and ASP.net are both good.
Simply speaking, PHP is easier to learn (IMO because of the absolutely MASSIVE number of free scripts available, as well as the easy to access, complete documentation).
ASP.net is no more "professional" or "industrial-strength" than PHP. The computer language you choose to write in is mostly a personal preference. Its like eating with either a soup spoon or a tablespoon. They are both spoons and they both get the job done.
Personally, I think ASP.net has won the MARKETING war, but not so much the technology side of things. PHP and ASP.net are both equally suited for serious corporate use, insofar as they both allow you to create large, complex, "enterprise-ready" applications. (Which brings up the question of what exactly constitutes "enterprise-ready". Neither language has any real advantage over each other, as they both have rather large featuresets.)
So in the end, I'd just look at cost/benefit, and pick PHP.
And even better is that so many people use PHP that its almost impossible not to find someone to ask questions to when you get conused.