the main problem with the cheap ones is that they are either on or off, so the temp gets pretty high then the fan kicks on and bang it's cold and the cycle repeats. with a setup like this the fans can slowly increase in speed to hold the temperature stable at a lower point. the units then usually run much cooler and quieter overall.
i hope that makes sense Pete,
I lack the programming skills to do this setup but i am using oz's setup similar outcome but hardware based with 2 pots.
Cheers,
Zogg