My memory is a bit sketchy on the details but I too looked at using Devi mats but on advise opted for just normal cable instead. I can't compare as such, but the cable was easy to lay around the bathroom and I would guess it offers a lot more flexibility than the mats.

I didn't use any decoupling layer, the cable is within a fairly thick layer of levelling compound laid over 22mm chipboard and with the water tightness layer and ceramic tiles on top. I have never had any issue with impact noise but as is normal here we don't were outdoor shoes indoors.

One area to really look out for is the controller. Ours is a basic one and a total disaster to program. You have to program the whole week day by day with all the various ons and offs and temp settings requiring multiple button presses. I should really change ours but it has not been a priority so I'm afraid I can't give advise on which is the best to go for. Perhaps look for instruction videos on youtube of various models available to you to see which is the easiest for you to program.

The worst is that ours lost all its settings when it was switched off over the summer even though it had power all that time. I couldn't face programming it again so it's been off all this winter. In reality I have rarely missed it. A mat or two in the bathroom takes away 90% of the need.

The tiles have been warm enough the last week that I would be turning off the heat to them now anyway.

My advise would be to not expend too much time or money on this, but do take a little care with choosing the controls. Unless you are very cold blooded you are unlikely to have it on for more than a few hours a day for 3 to 4 months a year.