Only buy BOTH of them together!
Here's the skinny. It is a Diablo killer. Period. That is what makes it great. It has superior visuals, great SFX, music, gameplay - all that made Diablo II so great is made better here. That also may be a fault - there is little originality here, but seriously, its not all that bad of of a thing.
The Immortal Throne should be played IN PLACE of the original game (not like it Warcraft III where the expansion pack is a separate entity) install both at the same time, patch Immortal Throne and get ready for the best Diablo-like experience yet.
A few noteworthy things - online play is rampant with cheating but it is ridiculously easy to play. Switching from Single Player to Multiplayer is seamless and does NOT require an addition character. The inventory system is intuitive, and the skill leveling system is incredibly customizable.
Basically, if you wasted hours on Diablo I and II years ago and still have nostalgia, play this game - it will win you over,.