What is more threatening to you as a Protection Warrior in PvP? Things you can block? Or things you can't? Your shield does nothing for you against casters. For that matter, even melee targets are going to try and get behind you making block relatively worthless. I don't see many rogues running up to tanks in PvP and just DPSing them in the face.
Resilience is a flat damage reduction, from all sources including magic, and regardless of which side they are attacking you from. Block is a damage reduction against melee sources that only attack you from in front.
As far as mitigation in PvP is concerned, resilience always wins.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.