hello my friend,you can use both of em if you are fast enough,and after all its personal preference , for example im used to single dagger so i use it,only way for dual daggers to bypass damage of singe dagger+cd is to make em +16.
I only use duals on macro with dual blow for the extra fatality blow and damage,then switch back to single dagger.
You can also use dual daggers for cancel buffs per normal hit,since they got more crit. rate than the single one,but again its preference.
About element it does not really matters,since in pvp (h5) server u will enchant skills+duel, so your element will not be added to your +duel blows(only if u go for + same element blows)
that means that element pays a role ONLY when u autohit someone,then again,as a dagger,u dont autohit,and if u do it,u dont do it for damage.
tldr? holy/earth is the best imo,but it wont affect things if u play properly.