A question of criticals

This is something I have been wrestling with for months, if not years...

Is it better to have a static percentage chance of a critical (a natural 20 is a 5% chance to threaten, regardless of who the attacker is) or make it a matter of degree? For example, if a target's base armor class without modifier is 10, a natural 20 is beating the target by an additional 10 (10 will hit on the nose, as will 11-20).

So a rapier, normally a 18-20 threat range, would threaten/critical on a +8 (getting a result of 18 or better against an AC 10). Of course, at low levels critical hits would be less common as armor class often outstrips attack bonus. At high levels, however, this would reverse itself. It would probably require a defense bonus to mitigate it a bit, so high level opponents squaring off don't just crit each other to death. I'll have to run the numbers, I think.

On the positive, certain maneuvers (like feint, trip, etc.) designed to make your opponent temporarily vulnerable would have a very real impact on combat. Feinting an opponent to make him flat footed against your next strike could very well end the combat. A trip attack puts a target in a very vulnerable position, making a wolf pack a very real and deadly threat.