yeah I have one of each and love my Thermostat.

A Rheostat acts as a dimmer switch - you control the amount of electricity actually flowing to the heating device. Thus if outside temps drop (lets say the A/C kicks on high for some reason) the device will continue to operate at the electric level that you set with your rheostat so unless you manually turn up the rheostat cage gets cold.

A Thermostat comes with a plug and a probe from the same unit. I personally put my thermostat probe next to my hot side probe in the hot hide so that I could monitor temps exactly where my buddy hangs out. The thermostat probe actually measures temperature and on the thermostat unit you set what temperature you want maintained around that probe. Thing is with a thermostat lights and heat pads or whatever click on and off repeatedly - to maintain heat round that probe, just like your heater or A/C unit kicks on to maintain the temp in your house. So bulbs don't last as long because they are constantly being turned on and off.

So to re-cap a rheostat is like a dimmer switch, you set how much power the heating device recieves to operate on and the rheostat maintains that flow.

a Thermostat is like the one in your house- maintains a specific temp around the probe.

I got my thermostat here (BAH-1000)
http://www.bigappleherp.com

If you have time to manually monitor temps and such a rheostat is great for you - as my schedule is scattered and I am sometimes away for weekends playing gigs on short notice, I use a thermostat.