Hi,
Nice question as you said already said all technology has advantage and disadvantage. When you started to compare technology wise between Asp.net and Php some will tell Asp.net and other will tell Php.

Advantage of PHP.
Opensource.
Support in all the server.
You can get all resource soon.
Anyone can develop.
php is Interpreter.

Disadvantage of PHP
No coding standard.
People who develop they have to develop their coding standard.
You cannt deploy the project.
You have to give source code to client.
If you facing any problem Opensource you have contact forum not any company to solve.


Advantage of Asp.net
Its Microsoft product license one.
Coding standard there in.
They having design and coding layer.
You can deploy project.
If you have any problem about the .net framework you can ask microsoft.
Once deploy all the code in one dll so coding side safer.
Asp.net is compiler.

Disadvantage in Asp.net
Support windows only.
Comparing to php code asp.net little bigger.
Its not opensource so you have to buy it.

this is the different i came to know about php and asp.net. If there is anything let me know.