A rheostat is good as long as the temp around it stays constant ... if the room temp fluctuates, you'll have to adjust the rheostat to account for the temp changes. If your room temp changes throughout the day, this could be a pain in the ace.

A thermostat is automatic ... it adjusts your temp for you ... you just dial in the temp you want and walk away ... the thermostat does the work for you.

Hope this helps.

-adam