I do not recommend PEXc it is simply PE pipe ran through an E-Beam to give it a partial cross linking.
   The multi-layer Pex, called Pex-Al-Pex some times is actually not Pex but simply two layers of PE tubing with a layer of aluminum between the layers. It is much cheaper to manufacture. It was developed to satisfy the plumbing trade, because of their resistance to plastic pipe, it bends and feels like soft copper. The multi-layer tubing has two problems, if it freezes it will not spring back like Pex does, it becomes over sized and will no longer accept fittings, if frozen more than once it will break the aluminum layer. It will also separate the layers when pulling through the holes in the floor joists on long loops where it must be looped through many holes.       

  PEX's long-term, high temperature strength depends largely upon the degree of cross-linking achieved during production.  If the cross-linking percentage is too high, the material may perform poorly at high temperatures and be brittle.  If the percentage is too low, the material will not fully develop the enhanced properties associated with PEX.  There is no simple standard for cross-linking percentages.
   The PEX manufacturing processes currently used in the U. S. include:
Peroxide cross-linking, Pex-a--There are several different processes which use decomposing peroxide to cross-link polyethylene.  The Engel process, the first peroxide method developed, sinters together a granulated blend of HDPE, peroxide and stabilizers under high pressure.  In this process, cross-linking occurs during extrusion through a long heated die.
   Silane cross-linking, Pex-b, --- the silane process is generally a three step process.  The first step involves grafting vinyl silane to a medium or high-density polyethylene resin to form a grafted co-polymer.  Next, this grafted co-polymer is dry-blended with a catalyst and extruded into the final product.  The pipe is then exposed to hot water or steam, which activates the catalyst and completes the cross-linking process in less than 12 hours. Pex-b is the most uniform of the methods available. 
   Irradiation cross-linking, Pex-c--Irradiation is the only physical process available for cross-linking.  In this method, pipe is extruded in a normal manner and then exposed to radiation through a high speed conveyance system.
   PEX is made to such high standards that there is very little difference between them as long as they have a ASTM F876/F877  certification by an independent testing lab and have a 25 year warranty that includes property damage.

ALL TESTING HAS SHOWN THAT PEXa AND PEXb WILL LAST OVER 100 YRS
THE  PEX SOLD IN LOWE'S AND HOME DEPOT IS PEXb AND IS VERY GOOD PEX AND WILL LAST AS LONG AS ANY OTHER PEX, BUT DOES NOT HAVE THE OXY BARRIER NEEDED FOR HEATING.

      Kitec pipes and fittings may fail prematurely causing leaks and water damage to your property. At least one expert evaluation has found that Kitec pipes have a manufacturing defect that can cause corrosion in the pipes. Corrosion can result in leaks and other problems with your plumbing system.
      Kitec consists of flexible aluminum pipe sandwiched between an inner and outer layer of plastic pipe (PEX Pipe). Kitec generally comes in two colors: blue (for cold water use) and orange (for hot water use). The pipes are manufactured by IPEX USA LLC and have been used in houses as far back as 10 to 15 years ago.
      Kitec pipe and fittings fail when they are exposed to water because of a chemical reaction known as dezincification, which results in reduced water flow and leaks. Dezincified  Kitec pipe and fittings cause damage not only when they burst and leak, but also impair the ability of a home's plumbing system to effectively provide water to appliances and fixtures.

O2 Barrier EVOH PEX pipe and your Radiant Floor Heat System:
Oxygen molecules will penetrate through the tubing wall and into the circulating water of a closed or open hydronic radiant floor system. The amount of oxygen passing through the tubing wall and into the system is well known for all materials and increases with temperature. For a typical system with plastic tubing working at 100F continuously, this oxygen diffusion leads to the generation of about 1 oz. corrosion products (MagnetiteFe3O4) per 100 ft. of pipe each. That is, if ferrous materials are present in the systems, such as steel or cast iron pipes, pumps or boilers. If the quality of the water is good, general corrosion occurs. The resultant sludge consists of a fine grain metal oxide that normally is easily transported with the water. It mainly settles where the water flow rate is low. However, the amount of sludge builds up over the years and will lead to circulation disturbances and other corrosion-related problems. With more corrosive water qualities, problems due to pitting corrosion are likely to occur early. Always choose an O2 Barrier PEX pipe for your Radiant Heating System