Is it worth equipping a 6 star memory on a character over a 5 star memory with set bonuses?

There is no cost to swapping them around so you should probably just put on whatever maximizes your BP currently.

Set bonuses are actually important. That def shred, that extra healing etc. can make your game alot smoother. Say, 2 Gloria is better than 1 Einsteina etc. It's better to wait and collect a 2-set of a 6* Memory. Just skip Condelina, it's pretty inferior than the other 6* Mems.