They are all pretty much the same with the AFAIK the only exception being High Guard changing to HG v2 in '81.
MM's CT cd rom has them all (except HG v1 for some reason).
I'd say it depends on your players. You're unlikely to get a group of players who are entirely new to Traveller (and if you do, you might be better going the Mongoose route) so they will be used to one version or another and a change may cause confusion. Having said that, most CT players have houseruled the basics beyond recognition anyhow, so in some respects it doesn't matter - whatever you can get hold of cheaply, which probably means the CTCD.
I didn't realise there was so many changes between '77 & '81. I would have started around '82 (?), which makes me the new boy on the block
Rules differences aside, The Traveller Book and The Traveller Adventure are what hooked me originally.
Not true.
CT LBB's printed pre 1981 have substantial rules differences from post 1981; the most obvious is that Jump Drives in pre-1981 do not require power plants. CT 1st ed is NOT on the CD-Rom.
CT LBBs printed post 1981 have the 2nd edition of the rules; there are subtle changes to damage and healing wording, unsubtle changes to personal combat damages, the Jump Drive change to ship design, and no route generation table.
The Traveller Book likewise has further wording changes to healing, combat, and some differences in the rules for space combat.
Starter Traveller makes changes to ship combat, using range bands instead of a miniatures approach. There are a couple of very minor wording tweaks elsewhere in the book.
Book 5 has two significant versions: 1979 and 1981. the 1979 HG 1E has a very similar design system to 2E, but substantially different combat rules, and different rules for generating the USP. HG 1E is not on the CDRom
The updated CDROM now has the 1977 LBB and the 1979 HG. It also has "The Lost Rules" which seems to lay out all the differences between the various editions (plus calls out a few rules from supplements).
Frank