Critique groups are great for getting typical reader reactions, and for spotting major flaws. That's certainly immensely helpful for improving your book!
But a critique group is much shakier when it comes to figuring out how to fix things. When a reader tells you "I read [Scene X], and had [Reaction Y]", that's an extremely reliable report; he's giving you a factual account of his own experience. But when he tells you "...but I would have reacted differently if you had made [Edit Z]", that's speculation. You will get good writing advice, suggestions, and speculations in workshop feedback, but you will also get bad advice, amateurish suggestions, and unfounded speculations. You'll have to navigate all that, and if the expertise of the particular group of critiquers is not so great - then you won't get much good out of them.
Bringing me to my next point - an editor is committed to your work. When I'm critiquing or reviewing a story, I don't need to fix it. If I think it's a lousy story, then "Your story is lousy because of X,Y,Z; I think it's fundamentally flawed, go write something else and don't make those same mistakes next time" is perfectly valid feedback for me to give. An editor, on the other hand, can't reject your work once they're committed to it (which is why good editors need at least some measure of care in what ms's they commit to editing).
That's really the fundamental difference: An editor is on board to make your work awesome; a critique group is on board to tell you what they think of it. So if a critique group makes a good case for your work being awesome, then maybe you don't need an editor. If the critique group makes a good case for your work being un-awesome, than you probably do.
(Now, you can try using critique groups as a polling mechanism: write, ask for feedback, then revise and ask for feedback over and over until your work has become awesome. But repeated "revision + amateur feedback" cycles don't necessarily converge into awesomeness - and finding new helpful reviewers over and over can be very difficult.)
Beyond all this, an editor can obviously be expected to be more thorough, to give better suggestions, to be available for numerous revisions, et al. But those, as you suspect, are more issues of "how well is my work edited?" rather than "is my work edited?".
At any rate, you should definitely start with a critique group - because they're an excellent and easy first step, even if you do wind up getting an editor later. See what kind of feedback you get; see how helpful it is and to what extent you feel that your next revision might be "publication-ready". Then you can take it from there.