Well...
It all depends the scenario...
For example:
A Warlock dots you and will do 1000 dmg over 20 seconds
So, every 2 seconds you recieve 100 dmg for 20 seconds.
If you have 5000 hp, you will gain 100 hp every 2 seconds.
This means you will totaly negate the dot and have full hp when the dot is over.
In the same scenario with Iron hawk at the end of the 20 seconds you would have recieved 1000dmg -15% = 850dmg.
Si here its clear... SB is better then AIH
BUT, if any can kill you under 16 seconds, AIH is better.
Because spirit bound can only provide you HP as long as you live.
AOIH will give you 5000 * 0.15 = 750 "extra" hp.
For SB to catch up, it will take 16 seconds ( 100hp every 2s = 16 seconds for 800hp)
So if a mage kan kill you in 10 seconds you will have recieved 750 dmg reduction from AIH increasing your hp to 5750.
SB would have only healed you for 10seconds = so 5 times 100 = 500hp. So your effective hp was only 5500.
So depending the situation or AOH or SB is better