I'd say stay away from Catapults. They're just not effective enough to warrant the time spent on them, and if you get balanced off to the other team you're effectively helping your enemies, right? Siege equipment stays in place as long as it's not destroyed, correct? I'm not entirely sure. I know weapon racks stay behind.
Anyway, I'd say in general that a Siege Tower of sorts is currently the most effective use of siege materials, due to the amount of hits it can take along with it's mobility. Catapult can be somewhat effective depending on the castle layout and whether you're using it on Attack or Defense, but dies fast, and a healing tent is paper thin.