Or an efficiency build, so that you can keep recasting it on the cheap. It really depends on how often you use decoy, switch teleport and radial disarm. I like duration builds because it gives me a longer decoy as well. But if I am going to be teleporting a lot, an efficiency build works better - depends on the mission at hand.
However, don't go for any negative duration mods - I just find they wreck Loki in general.