Most commercial differential controllers come with a whole array of other features such as: displaying collector and tank temperatures, setting maximum storage tank temperatures, pump short cycle protection, vacation settings, ... and more.  You can download the manuals from the suppliers of these controllers, and review the features and see what you think you need or want.

I'll pass on that I have our controller mounted in the bathroom, and I do look at the temperature displays for the collector and tank and find these helpful.  I also have the maximum tank temperature set to around 165F to increase the life of the EPDM lining.  I think its is also helpful to be able to set the difference in temperature between the collector and the tank that causes the controller to turn the pump on.  That said, I have used very simple differential controllers that only cost about $50 that did just fine -- so, this is a place you can save some money if the budget is tight.
 

Installing the Differential Controller

You will want to follow the instructions for the particular differential controller that you buy, but normally the install will consist of four steps:

1. Install a temperature sensor in the collector, and hook this up to the differential controller.  The sensor wants to be mounted where it will sense the temperature of the tube/fin near the top of the collector. 
   This temperature sensor will normally be supplied with the controller.
   For our system, I used silicone caulk to glue the sensor to one of the absorber fins right next to where the fin meets the tube.  This should be near the top of a riser so that it senses the temperature of the fluid leaving the collector.
    
2. Install a temperature sensor that senses the tank water temperature near the bottom of the solar storage tank.  The ideal location would be near the entrance to the U-Tube that leads to the pump inlet.
   For the differential controller we used, a  sensor was supplied that was suitable for direct immersion in the tank water.  This is convenient, as the sensor can just be suspended in the tank water near the entrance to the U-Tube.
   
   If you are working with a sensor that cannot be immersed, you can enclose the sensor in a length of pipe that is capped on the end and submerge the tube in the tank.  Use silicone to encapsulate the sensor in the end of the tube to provide better thermal contact and a additional protection against water.
   It may also be possible to just tuck the sensor in behind the EPDM liner, but you should do this before filling the tank, and be very careful not to leave any sharp corners.
    
3. Connect the pump to the controller.  Some controller just have a regular 120VAC outlet.  In this case, just hook up a line cord with a standard 120VAC plug on it the pump, and plug this into the controller.  If the controller just provides terminals to hook up the pump to, then run a wire from pump to controller being careful to follow the instructions for hooking up the line, neutral, and ground wires to the right terminals.
    
4. Connect the controller to a 120VAC source.  If the controller comes with a line cord and plug, just plug it into a wall outlet, and you are done.  If the controller does not come with a line cord, its easy to make one by cutting the socket end off an extension cord, and wiring this to the 120VAC terminals on the differential controller.

Some of the common differential controller brands are: Goldline, Steca, and Caleffi, and IMC.

John Canivan and Richard Heiliger at the JC-SolarHomes.com website offer a kit or assembled differential controller at a lower price.
I've tested one of Richard's prototypes and it did well and appeared to be well designed.

This Shem 32 is another imported possibility.  I've been using one of these on my Solar Shed system for this winter, and it has been fine.  Features similar to the Steca and Caleffi.  Price is attractive.

And this interesting myDTC one that looks kind of formidable, but does some logging -- would like to hear from anyone who has experience with it.

If you are using a PV driven pump, Guy Marsden offers a differential controller for PV systems ...  As does IMC (listed above).

Differential controller

There are a couple choices on the radiant floor heating system controls.

One straight forward system uses two thermostats in series. The first thermostat senses the solar storage tank temperature, and turns "on" when the solar tank water is warm enough to provide heat. The 2nd thermostat senses the room temperature, and turns on when the room is cold enough to require heat. So, when both thermostats are "on", the radiant floor circulation pump is activated, and the hot water from the solar tank is pumped through the floor loops.

This is the type of system I used on the Solar Shed heater project and is explained in more detail there.

If you want to use this kind of system but would rather use a regular programmable home heating thermostat rather than the Johnson A419 thermostat I used, this is relatively easy to do. Just have the home heating thermostat control a 24 VAC relay that switches the power on the pump line, as is done in the system described just below.

Since we added backup heating to our system, I am using a simpler system that eliminates the 2nd thermostat of the two mentioned above. That is, there is just a regular furnace style programmable room thermostat that activates a 24 VAC relay to turn on the pump that circulates heated solar water to the floor. The 2nd thermostat that senses solar tank temperature is dropped because the backup heating system insures that the tank is always hot enough to provide heat. The diagram for this system is shown below.

There is actually some merit in retaining the 2nd thermostat that senses tank temperature and keeps the pump from turning on if the tank temperature is too low to heat. The benefit would be that if something went wrong with the backup heating system, the 2nd thermostat would keep the radiant floor pump form staying on indefinitely trying to heat the floor with water that is too cool to provide heating. My thinking in taking the 2nd thermostat out was that we would soon notice the lack of heat and find out what the problem was and fix it.