I am positive nothing can beat the tetra preset heaters, any wattage.
I use the 50w in a gallon I think?
Large heaters don't overheat a pico, thats a misnomer. Several vases of mine in the past had 100w heaters. Wattage doesn't matter, only what will fit in your tank and has a thermostat.
you can't beat the preset 78 degree tetras...
I wouldn't use a pad heater either, just one of those tetras