Units: Delete or hide

I have a question I would love anyone's opinion on. My current project is set to have large long games with a huge number of units clashing in all of the fun and grandeur.

This however gets to the topic of what to do with dead units; does anyone know if it would be better to delete old units and remake them, or hide old units, clear their hate/target, heal them, move them to a respawning place, and unhide them to fight again.

I want to make sure I don't create game slowdowns from leaks.

Commander