The complex mathematics extrapolated from this excellent thread on Damage Calculation show how important defence is.
This also shows why defence down is such a powerful ability.
Defence is generally the more important stat in my opinion.
A good basic comparison is Scarlett and Kelile.
Kelile has low hp, Scarlett has low defence. I believe that Kelile is the tougher hero and a few more hp points on Scarlett do little to improve her durability
