The professional editions include things like resource editor, MFC, managed C++ etc. The best thing to do is to look at the Microsoft product descriptions and check which extra features you actually need.
One of these days I'm going to think of a really clever signature.
One of the big differences between the two is in the fact that you can't host plugins/extensions in Visual Studio Express. If you see a plugin that you absolutely must have, then you have to be using a paid version of Visual Studio.
*pre-emptive celebratory nipple tassle jiggle* - Sean Ewington