FoV is only available as part of DKS on the Xbox. It was originally intended to be made available as DLC, but ended up being too large for that, and Larian couldn't get a publisher for a disk release (only the German PC version of FoV got a retail release).
That should decide the question of switching to DKS, though you could finish off D2:ED and then play through FoV with a new character in DKS (in DKS you can start with D2:ED and play through to the end, or create a level 35 character at the beginning of FoV). As for the differences, there were a lot of generally small changes that added up to better to much better experience overall.
I played the Xbox version of D2:ED when it was released, and then got DKS when it was released, playing from the beginning with a different character build. If FoV had been available as DLC I may have just gotten that, but it worked out better getting DKS, anyway (for the changes as well as an included art book and soundtrack CD, which may no longer be available with new purchases, at a price not much more than DLC would have been).
