In general:
Grand Circle = ****
Vantage = *****
Viking = ******
For awhile last year I worked for one of those companies, and the efforts made to differentiate are pretty funny.
If you are booking through someone else, find out if they own the boat. You may be booking a luxury trip and find out when you get onboard that the company booked 50% of the staterooms (or 30% or whatever), and that it's not really a luxury cruise, just that you're getting a bunch of nice things added on.