Axe, mace, sword, claw or keep using my dagger? :3
Dagger. The only thing your offhand does is give you stats, apply white hits (weapon DPS), and apply poisons. There's a handy guide stickied at the top of this forum for Sub PvP that I highly recommend reading.
There definitely isn't a benefit from using a slow offhand as sub, if anything you would just be reducing your chance to apply poisons and you'd look kind of goofy :P