According to the tier list linked on the wikia, Grace is used more often than Sylseth for endgame content (majin 11-15) but Sylseth sounds amazing over Grace for every other kind of content. Grace also gives all of your allied melee units 10 MR. This can be very strong if you have Cynthia, one of the 1000 RC trading post blacks as she has a base MR of 40.