When it comes to web development these days, you have a lot of options but which of these 2 would you personaly use to develop a strong,professional,secure web application ?

PHP all the way! It's got nearly everything you would want from a language. It's open source. Put simply it is far better than ASP.NET

In terms of features you can do pretty much the same things in either language I think. But PHP is free, open source, with tons of tutorials and resources out there. Not to mention it works with both Windows and Unix.