That is almost too close to call. It would just depend on if you have any prior cruises on either line and if you intend to do further cruises on either line. Royal Caribbean has a wonderful loyalty program, as does Princess. I have sailed on the Brilliance and did a MEGA review of her that you can find here.
Maybe that will help in the decision.
Berlitz guide gives the Brilliance 1546 out of 2000 points and the Coral Princess 1544 points. This is like the dilemma of which flavor of ice cream to treat yourself too. You can't lose on either one.
Check the itineraries to see if one looks any better than the other and then select on that basis. If they are the same, try flipping a coin. VBG.
Have a great cruise!