Untouchable for BoL - You can always go in and out of the bubble to refresh the overshield. So Untouchable is more useful to reduce the cooldown.
Illuminated for WoL - If you're using WoL your main purpose is to increase dmg. Illuminated will boost it up that much more especially if your entire team uses it.
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The cooldown that it goves is verry minimal compared to what illuminated provides. Better of just running illuminated and run max intellect.