Differential thermostat ventilator/AC

This is a fairly basic idea -- if the temperature in your house (or room, in the case of a window unit) is hotter than the temperature outside, then the cheapest way to cool your house/room off is with a ventilation system -- a whole house exhaust fan, or a window fan.

If the temperature in your house or room is higher than the temperature outside, then the unwanted heat must be forced outside by means of an air conditioner.

I would like a device with a thermometer inside and a thermometer outside, which can compare the two temperatures and cool the room in whichever way is more appropriate.

Since I live in a rented room, I'd prefer that such a device exist in window-mountable form, similar to a window fan or window AC.