ALuminum PEX is usually used to increase heat transfer via conduction of the pipe walls to an outside material, often aluminum under floor plates. The pipe sits in channels in the aluminum plate and the plate is fastened to the bottom of the wooden floor to be heated.
The plate helps spread the heat out over the floor to be more uniform and radiate more heat from the tubing.
AL-PEX is more expensive and I don't of anyone personally who has used it for anything else. Typically you do not want the walls of the PEX to conduct heat unless it is infloor or an installation like mentioned above. When used as a transport medium, it isn't desirable for conduction, though copper and iron do as well.
Which relates to the other part of your question- what size to use?
First a quick visual explanation about how water in a tube behaves with heat. When in a tube, liquid (and to some extent gases) behave very differently than in open space. Think of how air moves over the top curved surface of an airplane wing. If you were to freeze time and mark the air particles in a vertical line, perpendicular to the wing, before the air reaches the wing, then skip ahead until the particles reach the other side of the wing, you would see (in a perfect situation) the particles all are lined up vertically again.
If you rewind just a bit to when the particles are encountering the wing, you will see those marked particles are still in a line, although the ones on the top aren't traveling horizontally in a straight line, they have to travel over the curved surface of the wing. Since a straight line is the shortest distance between two point, the curved path has to be longer.
In order for the particles to remain aligned vertically (if they didn't there would be a vacuum created in the middle of space, which doesn't happen in this situation), the particles on the longer (top) section of wing must travel FASTER over the surface. The faster a liquid travels over a surface, the lower the pressure on that surface, so viola, lower pressure on the top of the wing creates LIFT.
Okay, so what does that have to do with liquid in a pipe? Well, imagine the same air situation as above, but instead of looking from the side, look at it from the end. Now arrange a series of small wings all around the center circle (tube) of air, arranged like a pinwheel. You can easily see how the air traveling over the surfaces would indeed create lower pressure, but in this situation it would be in 360 degrees, so there wouldn't be any lift.
But...there WOULD be areas of lower pressure all around the outside edges of that tube of air. Now lets jump to water in a pipe with the same model. There is lower pressure on the inside surfaces of the pipe because of the water flowing over (past/through) the inside surfaces of the pipe. The water is held against the pipe by this low pressure, which means the particles of water sort of "stick" to the walls of the pipe. This creates resistance, which slows the movement a bit, which reduces the velocity of liquid over the surface, reducing the effect of the liquid over the surface, etc. See how these all feed each other?
These forces reach a balance. But in complete balance, water won't flow. So we apply an unbalanced force in the form of a circulator pump, which pushes water through the pipe. But remember the sticky inside walls? The water deals with these two forces by creating a layer of water that stays against the pipe, essentially creating a "sheath" of water that lines the inside of the pipe. It is easier for the water in the center of the pipe to slide/flow over its own water lining than the surface of the pipe. This is the friction of the pipe. It still does flow over the inside surface, but not at the same rate as the inner parts of the pipe.
This effect becomes more significant as pipe diameters increase. After all, with larger diameters, the volume of liquid in the middle of the pipe increases faster than the inside surface of the pipe. Looking at a cross-section, Area=3.14 x radius x radius. Circumference = 3.14 x radius x 2.
This accounts for the much greater BTU capacity of larger diameter pipe than smaller pipe, and is also why a small increase in diameter can yield a much larger BTU (think volume) capacity.
Another key factor is the BTU loss of the pipe. Remember the conduction we mentioned between the water and the wall of the pipe? The more surface area, the more loss. The effective BTU "delivery" is affected by the ratio of volume passing through the middle of the pipe without significant loss MINUS the loss from contact with the pipe wall (and the subsequent loss of that pipe wall to ambient air, other surfaces, etc). Additionally, the 'sticky layer' of liquid against the pipe has a bit of an insulative value, further reducing the heat loss in a larger diameter pipe.
This is why larger diameter pipe usually has LESS heat loss than smaller diameter pipe. It is also why larger pipe has LESS friction loss than smaller pipe. Ever try to suck a McDonald's milkshake through a coffee stirrer?
It's much easier through a larger straw, even though you are actually moving more material.
An example where all these forces are very well depicted is in Lava tubes. The lava doesn't melt the surrounding rock, yet the lava stays very hot and flows very quickly through these tubes vs flowing in open air. But- you never see very small diameter lava tubes! There is a balance it reaches between the temp/pressure and the tube diameter. It's really something to see!
...see next post...