FoV is the expansion, so that is all you need if you have the original D2:ED (and what you want if you wish to continue with the same character).
FoV continues seamlessly from the end of the D2:ED plot, and completes the dragon knight's story.
The changes made in DKS are mostly small things, that you will certainly notice playing, but if you don't otherwise wish to restart or try a different character build when you are done D2:ED, you wouldn't really need to get DKS (or install the conversion patch).
The biggest change would probably be the equipment system: merchants have better equipment in general (and some have good quality pre-determined items), though there is a little less random equipment in generic chests, enchantment effects have been increased and they are available sooner, and there are additional sets (some pieces can be bought). There are balancing changes which mean you should level faster at the start of the game (a couple people mentioned being a level or two higher when they got access to the full Broken Valley) and at the beginning of the fjords. Most of the anti-dragon shields were removed from the flying fortresses in DKS and they were redesigned. There were changes to the camera system and UI, as well.
If you don't mind starting a new character, or want a DVD release, then get DKS. If you want to continue with your current character, then get FoV, and then you have the option of installing the (large) conversion patch before the next time you begin a new character.
