The VS2008 version installs one additional code generator, but it isn't something I've put any time into in the last couple of years, so it isn't a major bonus at this point. There are not other feature differences between the two products. There are minor differences in text rendering because the underlying design surface switched rendering engines between 2005 and 2008, but it is unlikely you will notice a difference unless you print the models in both versions and hold them up to a light.
I'd go on price and other features that are not related to NORMA that you might want in VS2008 (it should have some additional support for SQL 2008). VS2008 also handles the tighter security models in Vista and Windows 7 better than VS2005, which requires some MSDN-provided patches to run well in these environments.