Which enchant on MagSorc weapon?

I have a 1pet MagSorc with 5x Necro, 5 IA and a Maelstrom Lightning staff. Run a heavy attack rotation. I have increase weapon damage enchant on my Maelstrom staff and run a shock enchant on my IA staff that does the heavy attacks. My question is what enchant is best on the heavy attack bar? I could imagine lightning being best but have seen a bunch of stuff saying fire enchant is better? Why is this? I've tested both but seem to get similar results really (on a target dummy at least).