The main hand/off hand items aren't determined by ilevel. :P You want the slower weapon with higher damage (not DPS, damage) in your main hand. This way your abilities hit harder. since they're the same speed that's not much of a consideration here. The Gurthalak has slightly higher damage, so it should be your main hand. But the difference would be pretty minor.
Warning: It has been a long time since I paid attention to DW DPS mechanics... You may need to ignore everything I just told you.
Edit: See... I knew I screwed up. Just noticed that your ESS wasn't the raid finder version. That would be the better main hand in this case.