Well, there are three methods to transfer thermal energy, convection, conduction, and radiation.
Convection is the transfer of thermal energy by transferring mass which contains that energy. An example would be the cold draft in the winter around a door, or the air moving in and out of a refrigerator when you open the door.
Conduction is the transfer of thermal energy by the molecules actually bumping into each other and transfering energy until they reach a point of thermal equalibrium when all of the molecules contain the same kinetic energy.
Radiation as heat transfer is the emmission and absorption of photons containing energy which effect the kinetic energy of the molecules and particles. An example of this would be a piece of iron held over a flame will become red hot, this is because it is emitting photons with enough energy to be in the red part of the spectrum. If you were to heat it hotter you would see its color change as more higher frequency (and hence higher enery) light is emitted as it heats. Everything radiates and absorbs photons to some extent, although at normal room temperature the energy contained in these photons is below the energy level that our human eyes are able to see. Another example would be the warmth that you percieve when you walk out into the sunlight.
These are the only mechanisms by which thermal energy is transferred that we know of.
Now in the case of your Pex tubing, hopefully there will be no convection, as that would be a leak of some type into or out of the tubing.
As for conduction, the aluminum, if in a metalic form would likely conduct heat better than the plastic, whoever with two layer of plastic on each side, and unknown thicknesses of each layer it is difficult to assess the impact. If the plastic layers in the tubing are thicker in the plastic/aluminum tubing than the single plastic layer in normal pex then the conduction shoould be less, if it is thinner then it would seem possible that it would be better.
That leaves radiation. Metalic aluminum tends to be a reflector of infrared light and is hence used in thin plastic foil type thermal blankets and such. It would also be rather reflective in the frequencies likely to be emitted by the normal temperature ranges of pex tubing in use, I would suspect that it would be worse at radiating and absorbing thermal energy. It would be interesting to see views of both types of tubes carrying fluids at operating temperatures as viewed with a thermal camera.
I don't know how much any of this might help you, but these are my thoughts at the moment on the subject, Rich
