How important is the Offhand for overall damage in PvP?

Hey, i was just wondering how important the Off Hand is for overall Damage. For example if i have an ilvl 239 Dagger in both Hands vs. an 239 Dagger in the Main Hand and lets say an 200 one in the Off Hand, does it affect the Damage of Spells alot or will the major part come from the Main Hand? I wanna know that because i'm trying to figure out if it's better to go for the better Off Hand or the 4 Part PvP Set bonus.

Talking about Sub PvP btw. for Mutilate i can imagine that it will make a huge difference

I've found it out myself now. For anyone who wanna know: Yellow Damage comes from Main Hand so Off Hand does only affect Yellow Damage with it's Stats not with it's DPS, therefore for Sub PvP i'd say Set Bonus>Off Hand.