Obviously I have a hard time getting into prae lately so my sample size is small but the couple runs I've noted I actually received substantially more exp/hr in castrum than prae. I dont have my notes on me at the moment but Prae is generally a 59% exp bonus and a 45min run time while castrum is around a 53% exp bonus and only a 25 min run time(off memory numbers). These were on my low 80s SGE and GNB.

So im not entirely sure where this "leave castrum" mentality stems from. I have heard complaints that its the amount of effort involved which I find silly for the exp loss and penalty but I've also heard the exp is worse in castrum which I have personally witnessed the opposite.