This doen't make sense.
The % is based on the # of enemies in aura radius, with a cap at 8. It wont matter whether partying or soloing. I would go with the aura Blind Chief reommended. My guardian only dies from phase and fire damage, rarely from a physical attack.

Maybe I didn't explain it very well? AoR doesn't scale in groups since it regens a set amount. So in a party as more people join the monsters deal more damage but you're just regening the same amount of health. No scaling. With AoD though it will scale. You'll have the same amount of armor, but that armor will be reducing more damage since armor mitigation reduces damage by a %. Understand what I said?

Yeah, the general tip I've seen is that AoR is the one for solo-ing, while AoD is the one for if you're usually grouped...