Nav Mesh Or Physics Actors?

Hi all. If I am running about 250+ simplistic enemies on screen at once. Currently they are made of a collision sphere physics actor that head towards a single location via impulses. Do you think its cheaper to have them run as these collision sphere physics actors or would a nav mesh AI actors be cheaper? Then when hit they could go physics rag doll. Mostly just trying to deduce wich is cheaper computationally. Thank you all for any help.