I'm not a huge "stat-watcher" , when selecting units I don't really bother looking at the numbers, my reasoning being ancient generals didn't have a statistical breakdown of their mens strengths and weaknesses. I do spend a lot of time reading unit descriptions. I have played many a Romani campaign though, just hit the Polybians in my current one, so feel qualified to venture an opinion.
With a Camillan army you go into any battle , even with odds up to two to one, confident of victory. You know your army will stand well, so long as you don't do anything silly. There is always a tiny doubt in the back of your mind though, those three or four times you have been shocked to see your Hastati rout, thinking "what the hell's going on".
You go into battles with Polybian armies even more confident, certain that, unless you do something completely moronic, your men will stand and fight. I can honestly say I have never seen a Polybian legionary run in any of my battles in a situation where I wasn't expecting it.