I do not have a VB background so I never tried ASP or later ASP.net. I cannot really comment on the relative strength of ASP.net over php. People have been telling me also that ASP.net is much better than ASP and ASP is slowly dying. Perhaps, the strongest point on part of ASP.net is its capability to seamlessly work between a desktop application and web based application through dll files. Which is of course, a great plus. Event trigerring is much easier and there are many built in visual tools available. You can transport C# and ASP.net in a flash.
However, I like php better perhaps, because that's what I know between the two. I have not run into a cul de sac with php so far. If I ever do perhaps, I may consider getting into ASP.net. Php's best strength is it is open source and free. Thus the greater number of people will always be using it. The php engine will be refined to perfection as days go by.
ASP.net is also expensive to install in a server. 60% of web server runs on apache because it is open source and free. And all apache servers have PHP installed. So getting a php, apache package is also cheaper.
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