I haven't read each edition in order to make a comparison. However, it's probably worth mentioning that, unlike the 3rd edition, the solution manuals for both the 1st and 2nd editions can easily be found online with a quick search. If you intend to sit down and start solving problems then the 2nd edition might be better, since you'd be able to check your answers against the solution manual.

Edit: I actually hadn't looked, but it looks like all the solutions for the 3rd edition are also available online [0].

[0] https://dreamanddead.gitbooks.io/csapp-3e-solutions/content/