Heavy onagers

[Oaty]

They bring down walls quicker plus 2 regualrs will probably make about 2 breaches for you where as 2 heavies might reward you with 3 breaches

EDIT: As far as accuracy and units it should be all the same and I do believe even minimum damage for both will/should kill elephants in 1 shot, whereas the ballista/arrow class boltthrowers usually take more than 1 shot unless they are modded.