I guess I am interested in knowing the differences between VSL's "Special Edition" instruments and their more "Pro" instruments. Is it the number of articulations? Is it the number of dynamic "layers" for each note of each instrument? Here is one question that I hope can be answered: Do the "Special Edition" instruments have samples that stretch over two or three notes verses one collection of layers samples per note?
All of this, yes.
The full libraries have a greater amount of articulations. However, a SE library along with its SE+ extension already offers a pretty good variety of articulations and covers quite a lot of musical ground on its own. You should definitely take a closer look at the sample content on the product pages of the libraries and compare SE libraries with their full library counterparts, in order to find out which of them suit your needs. For example, I think that for a lot of media/production music work, like music for television or games, the SE articulations can suffice. If one is mocking up "serious" orchestral works and very sophisticated music, the high detail and variety of the full libraries might be neccessary for the task.
Certain patches of the full libraries do have one more dynamic layer (some patches perhaps two?), which is of course nice to have. I think VSL has done a fine job in making sure that the SE libraries still sound expressive and musical though.
The SE libraries are sampled in whole tone steps, while the full libraries are sampled chromatically. To be honest, I don't really hear any significant disadvantage in the whole tone sampling. I'm working with a mix of SE and full libraries, and while I didn't actively listen for differences between half- and whole tone sampling yet, I can't say that I'm hearing anything in the SEs that irks me in practice. This might of course be different for someone else.
IMO, the greatest advantage of the full libraries, and for me the most deciding factor, is the greater variety of articulations.
P.S.: Sweet and spirited piece you posted earlier!