Depends if you play SBG or WotR of course x:
All 3 of the things you mentionned are good choices in both games though haha
I've got 1 ballistae, 8 Khazads and only 3 Iron guards myself.
I've been contemplating the idea of using Greenstuff to make a Mask on the Ranger models that have their throwing axes at the ready. It would make Iron Guards with capes~
Cheaper too and less heavy!
Won't have paint chipping off from gaming use.
Khazad guard models I quite like though, I wish they had plastics.. and the ballistae also can be made from plasticard and greenstuff, from this nice article:
http://www.blackbirdmedia.org/miniature ... rfBallista
Basically, buy Khazads ;D