Perhaps a ball of clay inside the wiggler would be best. That would simulate a developing chick inside an egg! And you can increase the size of the clay ball each week!
I like the water wiggler, simply because the water will better simulate the inside of an egg and will more accurately mimic the egg's ability to hold it's temperature (due to more similar physical properties). I borrowed my daughter's water wiggler, which she got at an iparty store. Other...
I just bought the same one for myself today! The "in" temp what the base unit is reading from the vent in the bottom (this is also where the humidity is being read from). The "out" temp is what the probe tip is at. If there is a difference in height of where the two readings are being made...