I believe the analog has a 3 degree differential, or at least I know mine does. I went with the analog as I am pretty sure the beer stays the 42 degrees that I have it set at. I am fairly certain that ambient temp in the fridge is going to drop 3 degrees before the beer does.
As for where I placed it, I ran it through the drain hole (I had to remove the compressor to do this) and along the back of the fridge to the top. I then placed the probe in a white labs vial that I had filled with the "blue goo" from one of the reusable freezer packs. Sealed the vial with silicon and used high strength double sided tape to attach at the top of the fridge near the thermostat.
Works great and I have never had any cooling issues.