Your question seems to be concerned with how well you can make a living with using PHP vs ASP.NET.
Regardless of your preferences regarding one platform or another, in your position I would first examine your local area and find out which one is more prevalent amongst the independents, then go that way, unless you have reason to believe that things are changing to more favor the other way.
As to a comparison between the two, do a search on comparisons, and keep in mind that some PHP types are incredibly biased against ASP.NET and may not know very much about it, and also keep in mind that the reverse is very much the case also. Here's some cases in point (with a commentary by me on how inaccurate the PHP-booster is -- I can't speak to the other, because I don't know enough about PHP).
PHP is used by many highly visible open-source users (Wikipedia, Wordpress and others), and I have found the many products which use it to be excellent. This includes a number of forums, and other kinds of social networking sites. The same can be said for ASP.NET, however. StackOverlow itself is ASP.NET (with a SQL Server backend). Here are some side-by-side comparisons, some of which are actually evenhanded:
http://www.bizfive.com/articles/web-design/comparing-php-and-asp.net/
This article is tipped in favor of PHP and asserts a few things about ASP.NET that are arguably false. A couple are: PHP is free and open source and there are lots of experts you can consult about it, as opposed to ASP.NET, which is closed source and you have to go to Microsoft to get support. Wrong. ASP.NET itself is proprietary, of course, but the run-time engine comes with Windows, and there are free Microsoft-supplied development tools that you can use to build sites. There are huge numbers of developers online in various social networking sites who love providing support, such as www.asp.net, as well as this one (SO).
http://www.soopertutorials.com/technology/programming/2475-aspnet-php.html
This one tips toward ASP.NET, I think a bit more honestly than the first one. But then, I'm not a PHP programmer!