I've used back heat in the past and currently use belly heat.

Depending on how warm and stable your room temperature is, belly heat is usually safer.
I used back heat for some time. And it worked great. It kept a nice temperature gradient and had my temps perfect. But my room never ran above 70-75. Since back heat affects ambient temp and air temp, the flexwatt ran at 115-120 degrees just to maintain the perfect temps. I was uncomfortable with the high temperature it was putting out, so I changed to belly heat. Flexwatt shouldn't run above 100. It starts becoming potentially dangerous after that point. With belly heat and the same exact room temperature, my flexwatt now runs in the high 90s to maintain the same exact tub temps.