It seems to me that you cannot easily compare these systems.
First of all the
Visual Studio 2008 Express Edition seems to be downloadable in pieces; whereas
Visual Studio 2008 Professional or Standard is a package of all the tools you need to develop any type of project in any language.
I've never used the Visual Studio Express edition for developing so I don't know what it's limitations are.
If you are a professional developer looking to develop an application for a company I would recommend purchasing Visual Studio.
If you are just looking to try out developing in .NET, by all means try out the Express edition.
Please note that if you are being contracted it is a good idea to have the full version of Visual Studio.
If you attend Microsoft or .NET conferences/conventions/user groups you may be able to obtain a free copy of Visual Studio (that's how I got mine).
-Frinny