So there are now two different editors that are out from Microsoft. There is Visual Studio which has been around for a long time and now a new tool called Expression Web. So which should you use?
Well it all comes down to what you are going to be doing.
If you are creating a lot of HTML pages and designing them and working with client-side scripting, then Expression Web is the tool for you. The entire Expression Suite is geared towards Graphic Designers. It has tools for everything from creating images, to creating web pages and even Silverlight or Videos.
If, on the other hand, you are doing a lot of work that involves code-behind (connecting to databases, calling web services and things like that, then Visual Studio is the tool for you. It can do the same stuff that Expression does, but since it isn’t focused on it as much, we can separate the two.
Hope that helps to clear up the story between these two. Please keep in mind that these tools are designed to be used with each other. So both can open the same files and make changes. This allows the Graphic Designers and the Web Developers to work together on the site at the same time which can really help enable your developers.