The temp shouldnt vary that much. Its a thermostat, and while its not going to be 100% accurate or as close to accurate as say, the herpstats, i rarely have fluctuations that are even worth noting in my rack systems that use it. I also use Herpstats. Love the things. Worth every penny.
Where is your enclosure placed? Near a heater, or drafty area? Near a window? Radiator? anything that would change the temp?
A Rheostat is going to be worse, and you will have to adjust the knob according to the temperature of your house/time of year. Much less reliable than the 500R T-stat IMHO.
Things dont have to be perfectly exact either. just so long as they are in acceptable ranges, you are fine.