It all really depends upon how often you tend to revisit the additional content and of course cost.
Personally, if you have the current edition of the discs then I would suggest holding off until you trade up to Blu-Ray and then when prices go down and budget increases you can trade up to the Special Editions in the improved format.
Unless you have money to throw about.
If you have not yet purchased then these days the price in DVD doesn't seem too different and in these cases I tend to buy the Special Editions.
But dont listen to me, I still have not traded up all the Trek movies from VHS with 4 out of the 10 in DVD and the VHS box set of the first 9...
DanG/Darth Gurden
The Voice of Reason and Sith Lord
“Putting the FUNK! back into Dysfunctional!”
Coming soon. The USS Ganymede NCC-80107
"Ad astrae per scientia" (To the stars through knowledge)