Coming from a classic ASP background I had the same questions. 3-4 years ago I took the route of moving towards ASP.NET/VB. I can tell you that the cross-over from ASP/VBScript to ASP.NET/VB is little to none. I was actually quite frustrated with the whole .NET platform for the first few months (more like the first year!) and kept rolling back to classic ASP.
In the long run, I ended up starting from scratch and picked up ASP.NET/C#. Oddly enough, I felt that the syntax of C# was more natural, even though my background was in VBScript!
For regular web development, ASP.NET is like using a sledgehammer when a simple ping hammer will do. However, the sheer power behind the .NET platform makes it invaluable in an enterprise environment where your web is often time blurred with your other applications.
Given what I know now, I would have likely made the move to PHP. Not only is the programming style similar, but PHP really is dedicated toward the web. Whereas it is quite easy to get lost in the mass of information the .NET platform provides. And the rate at which the new .NET techonologies have been coming out in the recent past can and has become overwhelming.
To directly answer your question: if you are staying in the realm of web development then I'd recommend taking a hard look at PHP for your new projects.