Just a quick check of logs from last week, looks like PW:S gets used up first:
Code:
[00:58:52.984] Asari casts Power Word: Shield on Azabuki
[00:58:53.203] Azabuki gains Power Word: Shield from Asari (Remaining: 32156)
[00:58:54.406] Azabuki gains Guarded by the Light from Azabuki (Remaining: 28801)
[00:58:56.875] Asari's Power Word: Shield fades from Azabuki (Remaining: 0)
[00:58:58.000] Shannox hits Azabuki 19014 (A: 5645, B: 10568)
[00:58:58.093] Azabuki's Guarded by the Light fades from Azabuki (Remaining: 0)
I assume there's a block in there that was partially absorbed, but you can see the PW:S fading 1.2 seconds before GbtL. I'll keep looking for more examples.
Edit: I found another example where the same behavior is reinforced:
Code:
[02:11:42.062] Asari casts Power Word: Shield on Azabuki
[02:11:42.078] Azabuki gains Power Word: Shield from Asari (Remaining: 31161)
[02:11:43.000] Voracious Hatchling hits Azabuki Absorb (16167)
[02:11:44.296] Voracious Hatchling hits Azabuki Dodge
[02:11:45.546] Voracious Hatchling hits Azabuki Absorb (18585)
[02:11:46.109] Azabuki gains Guarded by the Light from Azabuki (Remaining: 29509)
[02:11:46.500] Voracious Hatchling hits Azabuki Absorb (36962)
[02:11:46.593] Asari's Power Word: Shield fades from Azabuki (Remaining: 0)
[02:11:47.281] Voracious Hatchling hits Azabuki Parry
[02:11:48.093] Voracious Hatchling hits Azabuki 16306 (A: 26532)
[02:11:48.156] Azabuki's Guarded by the Light fades from Azabuki (Remaining: 0)
Edit: It occurs to me that this could be because PW:S is cast before GbtL in both instances. I guess I'll keep looking. Note that even with both bubbles and GbtL, there's almost never a situation where they're not both used up. GtbL in no way devalues a Disc priest.