It really depends on the application you're developing, and the level of enjoyment you get out of writing your own code. In terms of development, php will get you up and running faster, since asp.net is difficult to develop without the aid of Microsoft Visual Studio. PHP requires no IDE(Integrated Development Environment), but there are many text editors that are very useful:
http://www.textpad.comhttp://www.pnotepad.orghttp://notepad-plus.sourceforge.net/uk/site.htmIn terms of your application, you'll need to know more of the nuts and bolts (unless you use open-source pre-written code) in order to write the php, where asp.net (at least in terms of the basic display and data integration) is literally drag and drop with Visual Studio(VS). In both cases though, if you are going to get advanced, you're going to have to know a decent amount of coding. So really, the "best" language is going to be determined by your application's needs, and personal preference. Other factors might be: hosting location(in-house/remote), hosting-price(asp.net is usually more expensive), price in general (php and mysql hosted in-house are free, asp.net is free, but visual studio is not, nor is SQL Server, which plays nice with visual studio). Hope this helps.