if yo udont want the heaters to be seen, put them right in the wet dry. as long as they are somewhere where water is moving past them, it wi ldo the job its supposed to do. if you dont have room in the wet dry, i would place them in the tank, near to the return line, that way the return water is flowing near the heater. gives better circulation of the heated water, and avoids dead spots, gives a more stabile temp throughout the tank, etc..
notice, in the pic, my heater is right in the bottom of my wet dry and right in front of the return pump inlet. i now have it along the front wall of the wetdry, but it serves the same purpose, water movement over the heater.