Since Defense is so valuable for a Guardian tank, your BiS is generally 2 EWH passive Defense relics. No tank should *ever* use the EWH Shield relics: you get all the Shield rating you need on your gear naturally. If you undercut your Absorb rating a bit (some of the Guardians I know do this to get higher Defense, which generally pays out well because of Guardian's low Shield chance), you can get an excellent payout from the DG absorb proc relic.
I suggest PT/Vanguards use the shield relics since they can only get about 600 shield from enhancements and ear/implants. unless you are suggesting they use those slots for defense and absorb.
It has been shown that the proc absorb relic does not perform as well as the on use shield/absorb relic for mitigating damage.