Frenzy is a flat 8% damage increase regardless of anything else.
Sunder is only a greater-then-8% damage increase in specfic situations - Mainly when up against very high inf-armor targets such as Champions or Centurions with t3 armor upgrades. In most situations it's worse then Frenzy.
So Frenzy is the best choice.
To save you the effort of coming back once your Norse hits 40 - Adaptiveness, or +25% bonus armor, is far stronger then Parry, or 15% inf-armor. So take Adaptiveness