Buy the $20 one.
And, while at CF, do an advanced search for subject "thermostat" with the username "vettenuts".
Thermostats are tunable to some degree, and I know he has an excellent thread there detailing how to accomplish this and how to test it's operation before actually installing it in the water pump.
Any help or replies you may receive from him over there is quality advice you can take to the bank.
I totally understand wading through the
and hostility over there. There's still some really smart folks there too, it's just a matter of knowing who they are.