The original Dark Sun boxed set came out in 1991, designed by Troy Denning and Timothy Brown, and the revised box set came out 4 years later, 19951 designed by Bill Slavicsek. Both are for AD&D Second Edition. What's the difference?
First, we'll take a detailed look at the rule books that came with each set.
Appearance: They're both ninety six pages long, so neither is thicker than the other. Both are soft cover. The art of the original rules book is much better, in my opinion.
The original cover features the familiar Dark Sun logo. The artwork shows a Tarek charging an unknown foe. The land is desolate, a city is far in the distance, Ral, or Guthay, is rising above the horizon, and a creepy skeleton guy is watching what's going on. Painted by Brom, it's simply a wonderful work of art.
The revised edition cover features a new logo. I don't know why they felt the need to change the logo, because there was nothing wrong with the original. The artwork shows a Mul, Thri-Kreen, and, who I assume to be, the Wanderer fighting a Pterran riding a Silk Worm. There's some ruins in the background and a dark, cloudy sky with lightning crashing down. Painted by Stephen A. Daniele, it's a nice painting, but lacks the quality of the original.
Conclusion: The original looks better.