Buy the Blu-ray set.
Blu-ray players are now under $200. Also, even if you don't have an HDTV you can set the output of the Blu-ray player to work with a standard TV. That way you'll have the player and the best version of the show which is the blu-ray set.
Buy the Blu-ray set and you'll have both the original and new FX
And both the original and 7.1 sound
Also more bonus features than any other set by far.
Most of the new FX look great, some that they did first are a little weaker than the latter ones.
The old set (and 2-episode discs) were extremly dirty and had very poor contrast and color correction in comparison to the new restored versions.
Really it's just wasting money buying those old sets.
You know, I change my mind. I didn't realize blu-ray players were so cheap.
Go for the blu-ray.