One of the things about CoC is that it hasn't changed very much over the life of the game, so one edition is pretty much the same as another. I would look at the sample adventures included, as those have changed over the years more than anything else.
If you see something that's interesting, pick up that edition.
CoC is a great game to introduce people to, since the attitude of the investigator: ("there's a monster, run!!!") is easy to understand.