The main advantage to a UTH is that you don't have to worry about 'batches' having off-wattage. Nor do you have to worry about the wiring be secure and proper. Flexwatt most often comes without wires and if you don't get all that you need at once, another batch of the same size could give you different temperatures on the same thermostat. Flexwatt is fairly sturdy stuff, tho, and it's great when you need a bunch of heat sources (like with a rack).

CHEs are best for ambient temp boosts, since snakes do best with heat from below or overall heat from above as with a radiant heat panel. If you have the CHE far enough above the enclosure, humidity loss isn't as much of an issue but it still can sap the moisture. CHEs are best used with lizards or in rooms that stay pretty cold.