The Food: There is no question in my mind that the cuisine on Seabourn (calling it food doesn't do it justice) is far superior. Regent just does an OK job in this area. If you want a huge decent quality steak servedon a platter Regent can provide that, but if you want an elegantly prepared filet mignon served with understated flair, Seabourn is the way to go. There is really a huge difference in food quality, options and offerings. Silversea is also superior, though there has been a bit of inconsistency that Silversea is working to correct.
You hear that food is subjective. It is, but only to a degree. There are people that want to see a ice cream scoop of mash potatoes so that they know they are getting a full portion, as an extreme example. Some things - when dealing with luxury products - are not really subjective. It may well be that someone is not looking for a luxury product, but rather the product that they personally want...and then call it luxury.
The Service: I have never heard anyone say that Regent has equal to or better service than Seabourn...ever, ever, ever. No needing to find that great waiter. Seabourn has intuitive service (offering something you didn't even know you wanted) versus efficiently being given what you asked for.
The Itneraries: Honestly, I am not thrilled with the repetitiveness of some of Seabourn's itineraries. Regent does have some excellent itineraries. However, there are significant differences in that Seabourn can visit ports that Regent cannot and, for example, in St. Petersburg Seabourn docks right downtown, but Regent has to dock about an hour of of town in a commercial port.
The price: Don't get me started on this one! Regent (a privately held company) just made a very curious announcement that it has made its largest profits ever...EVER!!!! Couple that with the fact that Regent admits it has the highest prices in the industry (supposedly because of its now including some tours - though many are no longer luxury styled) and the math is easy: Regent is no bargain. High prices do not equate to luxury. True luxury travelers are willing to pay for quality, but not just pay to say they paid the highest rates.
Where is "the magic": That is the real question! Douglas Ward (Berlitz Guide to Cruising) is a fantastic resource for quantifying numerically the quality of a cruise line. What really makes the difference in the luxury market is how many times you say, "WOW!" whether it be a bar waiter arriving with champagne while you soak in the whirlpool, or an impromptu wine tasting, or a wonderful note from your stewardess, it needs to be something a whole lot more than remembering your drink or name. I rarely have experienced or hear of Regent having that magic, but on Seabourn the comments are frequent.