Ali-I allways overboard with Hardi but was only going with insulation board for the the thermal properties.
I don't think there is much difference between the two mate 0.00something.
Over a timber floor though I would probably opt for hardie. Yes marmox is easier to handle, cut and fit. I've just done a bathroom with UFH, chipboard floor and went over with HBB and I'm starting another tomorrow where ill use the same.
Thats just how I roll! But if its marmox you intend to use and its loose wire youre fitting im sure you could work the wire round the washers. A little less depth on the SLC too.