So which one is looking good for PT/VG's?
It depends on the comparative levels of your Shield/Absorb to your Defense. If you've got lower Defense rating compared to your Shield/Absorb, you'll get more out of the Defense relics thanks to the Defense contributions getting hit by DR less (for most VG/PT tanks, this should be true). If the opposite is true (factoring in the proc relic you're probably already packing, which adds a *lot* of Absorb), then you'll want to use the Shield/Absorb one.
If you're actually looking for the best numerical contribution over time, however, your best bet is actually the Campaign Shield/Absorb relic since it's the highest rating use tank relic with a 30 sec duration (the Camp use Defense relic is 20 secs and both of the DG use relics are 20 secs as well).