I'd love to see a barbarian type use a really powerful two handed weapon.
I go ahead and play that barbarian type, and who cares if it isn't as effective?
The primary value of the two-handed weapons in this game is the reach, not the damage delivered. In some ways, a big claymore or warhammer is best suited to a small, agile character, like a Bosmer or Breton.
But, in answer to the original question, a sword/shield is quite a bit more effective. Block is actually a very effective skill if used correctly, and a shield does add a bit to armor rating, as well as being another enchantable item. And one-hand swords are not that much less powerful than the two-handed.
If that weren't enough, the two-handed weapons get no sneak-attack bonus, which makes no sense at all.