Soren IVs

The +atk one....just remember that hes not really worth the divine dew.....except for the memes

The Hp bane look better, however remember that Hp allows you to survive both physical and magical and Res magical only

+SPD Is better for him, but not if you're planning to run ploy skills.
(And it's totally worth the dew, IMO. I ran the calculator and a buffed blade Soren is within 1-5 kills of a Wind brand+eff Soren)