I did a detailed comparison of Regent, Silversea and Seabourn on my blog in September. You can read it here
I would add that I think Crystal is also superior to Regent as its cuisine, service and enrichment programs are far superior. However, unless you book a suite, the cabins on Crystal are small.
Back when Regent was Radisson, the product was much better with a higher level of service and food quality. With its present "free, free, free" program it has IMHO become an expensive alternative to a premium or mass market cruise and simply not in the same league as the other three.