Cat Archers are pretty much Cataphracts with a tad less defense and melee capability. They are still tanks. You should field both because the charge capability is much stronger for straight Cats, and this makes them better anti cavalry units. However, the defense is so high on both of these that you can very easily survive anything short of elephant attacks with Cat Archers. You can use them as heavy cavalry, just avoid frontally charging legionaries of any kind, and of course, spearmen.

Cat Archers are unbelievable units if you use them in groups of 3 or more. You need not run away from enemy cavalry with groups like that. Just turn off missile attack and ALT double click to countercharge and you will destroy any enemy cav units. I've often won battles with only 2 groups of 3 or 4 Cat Archers, exhausting their missiles and then running down whatever is left. The rest of my army sometimes didn't even engage as my Cat Archers were way out front, "taking it to them". Focus on enemy missiles first, then cav, then infantry units. If the enemy is full of heavy spearmen, use your missiles in skirmish mode and retreat behind your own infantry and foot missile units. When Cataphracts are available, 1 unit for escort of each of your Cat Archer packs is all you need, and maybe 1 in reserve with your general behind your center.