I hear a lot of brewers say the temp control sensor needs to measure beer temp, not ambient. I'm concerned that if I measure beer temp the temp will continuously fluctuate the full range of the temp controller, which for me is 3 degrees F. I let the sensor measure the air temp. The temperature of the beer will change slower than the air temp, so the beer is "chasing" but never catching up to the air temp and will therefore fluctuate over a smaller range. Tom Hampton says the fluctuation of the liquid will be about half the fluctuation of the ambient air.
Tom says he gets better results measuring beer temps, but he has a much better - more sensitive and more expensive - temperature control system. If I were willing to invest in a system such as Tom uses I'd probably measure beer temp.
I'd recommend considering the temperature range you can control before attaching the temp sensor to the fermenter.
Am I missing something?