The Viking can replace the Knight as the heavily-armored warrior of choice after the Water Crystal. Virtually all of their weapons are Lightning-elemental, making them especially effective against sea creatures. However, Vikings suffer from low accuracy, which can make them suffer offensively. A dual-wielding Viking will deal a lot more damage at the cost of defense.
Cid is a pretty basic physical attacker. His Study ability is like a free Scanspell, but it's rarely useful. He is one of only three characters, along with Cecil and Kain, that can use shields, but his primary weapons (hammers) are two-handed so this is not very useful. There's not much subtlety to what Cid does.