I will let the Anova people do most of the work, but if you are thinking of using a big cooler, I would say don't buy the Nano, the clip does not seem to be adjustable for a thick walled container. While there is a difference between the bluetooth and wifi units, they both can do the job, so it is more about how you want to connect. The wifi gives the most options for connecting.
I own the bluetooth and it does fine in my 24 quart cooler. I don't connect at all, I use the controls on the unit.