Just want to say that I absolutely love the Zweihander! I'm still new to this game(around 40 hours total). I messed up my first character(30 hours playtime) and decided to start over and make the Zweihander my main weapon and so far it has exceeded my expectations. I've read about people adding lightning or fire to their weapons and was wondering what the best course of action should be? I have it upgraded to 10 and I'm close to the giant blacksmith. Should I just go ahead and make it an elemental weapon or up it to 15?

TL;DR: should I make my Zweihander an elemental weapon or make it 15?

EDIT: I want to add that this is for PvE(my gold ran out this month and I don't have a desire to renew it just yet)