I know I have only been on Crystal and Silversea recently. I have not been Regent, just a line called Radisson many years ago ;-)
I will take Crystal's food over Silversea's. I thought the food on Crystal was just plain better quality; prime cuts and fresher. It is also more "American" with steaks and lobster, rather than veal and fois gras in aspic. I admit, I prefer that.
The dining time thing is a bit of a nuisance, and Eric is a better judge of the lux lines that I am - but I do think some people just prefer traditional cruising and they don't mind fixed dining times.
Open seating often leads to slower service, if you get a rush of everyone at 7:15. Crystal is crewed for two seatings, and something times those things are harder to change than you think.
I will never forget my butler on Crystal - Papa. He was GREAT! He made my butler on Silversea look like an amateur. He always brought us full meals and set up the table with a table cloth and linens.
One time he brought us a caviar snack that we just devoured - with cheese and chocolate strawberries. We were too full to go to dinner.