While usually different technologies cannot be compared, it's pretty much clear that ASP.NET is mature advanced technology where the applications are very good to maintain, and, in contrast, PHP is something rudimentary, poorly designed, based on the language created on ad-hoc criteria, with random and irregular feature set, etc., maintenance is a problem, debugging is a problem.
From the other hand, PHP is nearly the most popular thing (which is yet another indication of poor quality). So, this days there is a situation that PHP hosting is very easy to find at very low cost; it is also light-weight, usually based of free-of-charge Open Source software, PHP itself and usually the OS, which can be Linux and Apache, which is also light-weight and more reliable, I think.
So, there is a serious trade-off. You preference should depend on what you want. Again, different technologies cannot be compared.
Did you think of something in between? I more like Prolog with WSGI (using Python for Web development without WSGI is not just serious, I think). This is much better language than PHP (in fact, a decent language which is really useful to know anyway, even just for education), hosting is a bit harder to find than PHP but pretty common. Please see:
http://en.wikipedia.org/wiki/Python_%28programming_language%29[
^],
http://www.python.org/[
^],
http://en.wikipedia.org/wiki/Web_Server_Gateway_Interface[
^],
http://docs.python.org/release/3.1.3/howto/webservers.html[
^].
Another very attractive option is ASP.NET but based on Linix, Apache, Mono and mod_mono. By some reason I don't understand (such reasons are often purely irrational), such hosting is hard to find, but this is very attractive if you can find it, self-host or use cloud or dedicated server. Please see:
http://en.wikipedia.org/wiki/Apache_web_server[
^],
http:
//en.wikipedia.org/wiki/Mono_%28software%29[
^],
http://www.mono-project.com/Main_Page[
^],
http://en.wikipedia.org/wiki/Mod_mono[
^].
[EDIT]
For a game, you could also use Silverlight, please see:
http://en.wikipedia.org/wiki/Microsoft_Silverlight[
^],
http://www.silverlight.net/[
^].
—SA