I haven't read the comics in quite a while,but Venom was always much bigger than Carnage and when you take a look at who's under the symbiote,Cassidy is pretty much this skinny guy,while Brock is built like a weightlifter.
I would give it to Venom. Carnage is a killer,yes,but when you're insane like that you aren't really thinking too much,just reacting. All you want to do is kill. Brock,on the other hand,may not be a brilliant guy,but he's been in this symbiote longer and has more experience than Cassidy and has a bit more smarts. When you're reckless like Carnage bad things can happen.