ASP and PHP are similar in that both tend to put their code in with the HTML, and so the logic can be quite similar.
But, ASP.NET will be very different from PHP in design, as there is a strong incentive to use code-behind in ASP.NET, where you basically have the html template and all the code is in another separate file
Depending on what you are doing, how busy your site is, you may find that the speed difference is inconsequential, though one is compiled and the other isn't.
PHP is probably going to be faster to develop, as you can more easily code a little and test, than you can with ASP.NET, but ASP and PHP are similar in how you can develop.
If you don't know any of these languages then PHP may be the easier one to learn, as the php manual is so well written, with lots of comments from users, and ASP.NET has replaced ASP, so learning ASP for a new project, IMO, is of limited use.
If you go with ASP.NET then you are learning a new syntax and one of the .NET languages, but depending on your background, C# may be relatively easy to learn.
With ASP or ASP.NET you are stuck with using IIS for your server, but with PHP you can use IIS or Apache, so there is considerable flexibility there.
With ASP.NET you will find more options to help with code development, as they now have the classic ASP.NET and ASP.NET MVC (http://www.asp.net/%28S%28d35rmemuuono1wvm1gsp2n45%29%29/mvc/), both with pros and cons, but I believe this site is still written in the latter.
So, which would be better depends on what you are going to be doing with it, and what languages or frameworks you have already gained experience with.
