I've never used the 36v DeWalt.
I just looked up the 36v platform and all I can see are 2.0Ah batteries. If that's the case, the 4.0Ah - 18v batteries have the same amount of power at 72Wh. I'm pretty sure that's a brushed motor so the FUEL should have the edge.
I literally can't imagine wanting either more power or run time than my 6.5 FUEL circ saw provides. If I wanted more run time, I could always load it up with a 9.0Ah battery so I could build three or four decks between charges.
Hey, I'm not saying the runtime on the dewalt is good, it's not bad, but not good. But the power/torque difference is night and day. IMO, the battery tech isn't up to the task of making a "real" circ saw.