Also Prae has the highest poetic output out of all these, so if someone's running it for poetics, that might be the reason if they don't have access to or don't know about The Aetherochemical Research Facility. I know as a sprout when I had no means of gaining poetics I queued for prae and watched smth on the side (alternatively passed out during Nero's speech).

But yeah if you want tomes and don't want to suffer much research facility might be better, if you want that juicy msq roul exp, you just take whatever it gives I guess. You'll only get Castrum more if there's a moogle tomestone event going. This is my observation anyway...