Carnosaurs all the way, have more versatility than the steg. While it might lack the mortal wounds offered, the carnosaur makes up in a few ways, notably its faster, better bravery means heroic recovery and other utility, the carno also has the ability to deal more damage, and finally we come to cost. With Stegs being so much more expensive, the fact that carnosaurs can equal, if not exceed their damage output is a testament to carnosaurs being lethally efficient and being a better melee damage platform than the stegadon.
IMHO, if you're taking a stegadon its for the ranged weapons, or the potential mortal wounds they can offer. Simply put you dont take a stegadon for the raw potential melee damage.