I use a sensor at the outlet side of the radiator A) because I had this radiator and B) because this way the coolent temp entering the engine will remain more consistent. The thermostat sets the minimum temp of the engine, and the fans set the max temp, but I prefer to defer temp regulation duties to the thermostat whenever the choice can be made. Sensing at the stat housing places more of the burden of temp regulation on the fans--not saying that's worse; I personally prefer not to place the ultimate temp regulation responsibilities on electric fans (more can go wrong with them than can with a stat). Sensing at the outlet of the radiator also means that if the natural airflow through the radiator is sufficient, the fans won't turn on unnecessarily.
Yes, there is latency when sensing temp spikes, but the water pump keeps things moving quickly enough that temps could ever spike/swing that greatly if at all, and the fact that I'm using a 195 instead of trying to bump my operating temps up with a 205+ stat means that I've got more wiggle room anyway. Likewise, there is a latency problem with mounting the sensor at the stat housing or similar--instead of having a latent reading of temp spikes, there will be a latent reading of temp DIPS generated by the fans turning on or by exiting a traffic jam. In a way, I like the outlet location simply because my fan controller has more direct control of the fans this way--logical.
There are pros and cons to any sensor location. There's no such thing as a perfect spot as far as I'm concerned.
Build Thread -- http://www.jeepforum.com/forum/f177/whats-chrokeese-build-thread-2525690/