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I may be too late but i'd like to note that from my experience the best way to go is to make your offhand weapon revolve around an increase in dps and/or attributes and bonuses that would benefit.

My thief friend has tried out various daggers varying in damage and delay while used offhand and has noticed little to no significance in WS damage change. Leading me to believe an off hand weapon's main purpose is strictly for DPS, WS enhancement or side bonuses that would benefit any other activity.