Is blocking with a one-handed weapon equivalent to blocking with a shield or a two-handed weapon?

I'm not a huge fan of two-handed swords or shields in Skyrim; I'd like to just wield a one-handed weapon (dagger, sword, etc.), but I'm worried that my blocking won't be as powerful and that I'll take more damage.

Am I worried for nothing, or do I really need a shield?

According to the in-game book "Killing - Before You're Killed" by Eduardo Corvus, blocking with a weapon is less effective than blocking with a shield:

You can also block without a shield as well - just cross that blow with your sword, though this isn't nearly as effective. And if you're fancy and want to wield two blades, you can't block at all, so don't even try. Without both hands on the hilt you won't have the strength to counter blows.

According to the Elder Scrolls Wikia page on the Block skill, you also cannot block with a spell in one hand and a weapon in the other.

It is possible to block with one handed weapon as long as your left hand is empty. You cannot block while dual-wielding.

Blocking is slightly less effective with a weapon than with a shield. With 0 blocking skill, a weapon blocks 40% of damage while an iron shield blocks 50% damage. At 100 blocking skill, a weapon does 60% damage reduction, while a daedric shield does 63% damage reduction. Blocking with weapons benefit from the shield wall perk at the bottom of the block tree.

Bashing with a two-handed weapon does more damage than bashing with a shield. With deadly bashing perk, daedric warhammer does 102 damage, while daedric shield does 68 damage.

You cannot block arrows with a weapon.

Source: http://forums.bethsoft.com/topic/1342463-complete-character-design-freedom-damage-resist-caps-and-ridiculous-damage-thread-10/