Here's a quick overview of the editions, including some notes as to how they differ from each other.
[ADDENDUM: 26 June 2012] In the spirit of the game, let me take a moment to issue a correction, and point out that one of the biggest problems with MegaTraveller was the immense amount of errata and corrections that were issued over the six years of its active publication. Most of those have been consolidated, and you can find them, for instance, here. There is also errata for the other editions there, but none of them approach the extensive errata issued officially for MegaTraveller. Well, the errata for T4 comes pretty close, I suppose.