dual input temperature comparison circuit
Here's a simple circuit that should do the trick. It's a kind of Wheatstone bridge with a comparator. The two transistors should be from the same lot- they'll typically be matched to within &lt; 1°C. If you need to trim them so they're exactly balanced, you can trim R5 or R6.

Take care as to the layout- the path from M1 back to the supply should not go through any of the analog circuitry- millivolts count. 







simulate this circuit &ndash; Schematic created using CircuitLab

Edit: Since you want to have a non-zero nominal differential, you'd adjust the resistance R5 or R6 to get the differential (for example, you could replace R5 and R6 with 1.8K resistors and connect them with a 500 ohm pot instead of directly, with the wiper going to the + input of OA1. There will be a small effect on the differential with absolute changes using this circuit. You would adjust the difference to about 2mV/degree C differential, so if you wanted 5°C differential you'd adjust the pot to get something like 702mV/712mV from the + input to ground vs. the + input. If the absolute temperature changed 25 degrees C then the differential would change by about 7%, so about 0.35°C out of 5°C. 

Edit2: 

Photo of transistor test: