The guarantees are not really what they seem to be. No one can guarantee that you will get a certain score.
I have heard decent things about Testmasters. They are one of the major players and well-respected.
My recommendation is to take a live course if you can aford it. I think that live instructors can give you a different kind of personalized feedback that you can't get online. How much extra is that worth--I really don't know. How much difference will it make--I don't know that either. But, I think that it isn't unusal for a live instructor to catch a weakness that the online course wouldn't identify. If it gives you 1 or 2 points, it could make all the difference.
Good luck, and let us know what you decide.