Defense Score VS. Defense Roll

As I've mentioned before, I've been toiling away on my own roleplaying game called Dungeons Deep & Caverns Old (a incredibly imaginative name, isn't it?). Recently, I've been working on the game's combat system and determining what to include and how to string it all together into a cohesive whole.

At the moment, the combat system works like a more stream-lined version of D&D combat. When attacking someone, you roll a d20 and add the appropriate modifiers to the die's result. If that final total is equal to or greater than the opponent's Defense Score, your character hits and rolls damage. However, due to my recent reading of Hackmaster, an alternative has occurred to me: Defense Rolls.

Instead of each character having a Defense Score, they would have a Defense Bonus. During combat, the character would roll a d20 and add their Defense Bonus to the die's result. If their final total is greater than the opponent's attack roll, they successfully defend themselves and aren't harmed by the attack. If the two rolls match and result in a tie, the character with the highest bonus succeeds.

I find this alternative attractive for a handful of reasons. 1) Having players roll to defend against attacks keeps them engaged throughout the combat, 2) It would allow me use Armor as Damage Reduction, which is another alternative I prefer, and 3) It would allow me the option to have the players make the dice rolls during the game (I would just take the NPCs Attack and Defenses Bonuses, add 10 to them, and have those be the numbers a player has to roll to successfully attack someone and defend themselves in combat).

While I'm really tempted to go with Defense Rolls, I'm wondering how I should handle shields. Should they just grant damage reduction like armor, or should they grant you a bonus on Defense Rolls. I might try both out and see which one works the best.

What do you guys think? Do you like the idea of having to roll to defend, or would you rather have a static Defense Score?