Heat mats generate heat over their active area. I think the risk is, if some areas break you could end up with the power concentrated in remaining areas and they get relatively warmer. I've heard of suspected fires started by malfunctioning mats in the reptile community. It would be safer therefore to stick them to the side of the tank where they wouldn't be under a load. Some come with adhesive on them, although there will be a challenge of how to make that look good. Also note they don't have any thermostat with them. You'll need to find a separate controller to regulate temperature. There are on-off ones available. I have previously asked Microclimate if their ministat 100 (up to 100W) sensor was waterproof, and they said it was resistant but not designed to be continuously submerged, and suggested putting it in some waterproof coating first. I was looking at options for a low cost 2nd line of defence as I've seen a few horror stories of heaters being stuck on killing the tank recently.
Depending on the room you put it in, and how you set your temperatures, the previous thought of not running any extra heating may be a consideration.
I think I've seen heat cable too, but I haven't any experience of this.