Are you going to fasten the tape to the bottom of the tubs? Or are you going to fasten the flex watt to a surface and lay the tub on it?

If you are directly heating tubs I'd recommend against on off types. I don't know much about geckos but the on offs all have the same specs. It will run over and under the set point by one degree. (set at 90 it will run from 89-91) The other spec that matters is the probe accuracy. They are spec'd at +/-2ºF accuracy so in the worst case it could run 3º over the set point. The other end of this equation is the speed of the change. They typically run at 1 min changes That means the animal is subjected to the full power full heat for that period of time.

If the heating element is not fasten to the bottom of the tub that will leave a small air space and the effects of all of what I just mentioned are mostly nullified.