As for the difference in stats... I think we need to compare actual weapon setups to each other (including enchants). We're never going to have full flexibility of secondary stats in a given tier, so that can strongly influence the 2H vs DW choice.
Speaking of enchants, do we know the cooldowns for the procs? If we had DW and colossus had a single cooldown (as opposed to a cooldown per weapon), we might want to have something like windsong on our OH (which seems to be 3 separate procs for each stat that can overlap).