Defense reduces the chance for you to be critically hit as well as increases your dodge, block, and parry stats at the same time. Resilience reduces your chance to be critically hit and reduces the amount of damage taken from critical hits and DoTs. Basically having high defense and resil together will make you harder to crit, while maintaining the majority of your tanking stats. The issue that you run into is that if you are a "tank" in pvp, then chances are people are going to notice and generally try to ignore or CC you. If you do get focused really hard, the resil and def together will keep the damage lower, giving you the ability to last longer, however more often then not the tank will get overwhelmed after a certain period of time.
Losing the damage stats on pvp gear and gaining the avoidance ones on tanking gear are only worthwhile in certain situations. Usually a tank is hard to kill with or without resil due to their high health pool and def rating already. It would be more effective to have higher end pvp gear, with pve dps gear while keeping roughly the same amount of survivability. Keep in mind as well that resil scales alot better in crit reduction then def rating does.
"If I'm doing a fight and I need more threat... I try harder. If I'm doing a fight and I need my taunts not to miss, then I wear hit." -Veneretio