Hi Markus,
the download version of Extremes won't probably be smaller than SS08. I'd say probably a bit more, as there is new content.
Anyway, the file will undoubtely be several gigas to download. If your internet line is good enough to download some gigas, wether it's 4 or 5 doesn't change a lot ...
About the disk version been easier to install and letting you keep a hard copy, I don't really see the point here - both versions are equally easy to install, and you still can burn on a DVD the downloaded file.
The real advantage of the boxed version ... is the box and the materiel it contains
The advantage of the download version .... is that you'll be able to sail Extremes a few days sooner.
I imagine there might be a slight difference in the pricing (the download version beeing probably a little cheaper than the box one - at least you have no shipping cost).
Regards,
Denis