my mention of the AX type belt is as follows
an A belt is a common belt with a solid body
and AX belt is the same size (width and length) as an A belt, however
it has its underside notched so that it can wrap over tighter pulley diameters and it also
has the ability to handle about 40% more power than a standard A section belt.
i use dual AA belts on my larger unit, changfa s195 and st7.5
they are basically hexagonal in cross section allowing them to drive from either the top
or the bottom of the section, which is quite useful for serpentine drives.
i chose to go with them based on the design of the unit and my not wanting to have trapped belts
which would necessitate removal of my direct drive coupler system to the st7.5
the AA belts drive dual 110-555jho alternators and a sanden A/C compressor
they do not slip or even run much over about 100 degree's on a 70 degree ambient day, this at full rated load on the engine which equates to about 7.5kwe
i think the lowly V belt has gotten a bad wrap from folks that insist on using stamped steel pulleys, misalignments, or worse cheap belts and pot metal pulleys that are no where near up to the task at hand.... when they don't work well the belt system is always to blame or so it seems.
now having said all this
there is much to be said for the poly groove belt, mainly because they are so much more widely available these days just about anywhere, whereas v belts are not so well represented in the autoparts stores. then there is something to be said for economies of scale, choosing a very common automotive belt usually means the lowest cost to the end user... something like using a belt made to fit a small block chevy maybe or whatever is the equivalent these days.
don't forget the hd truck dealers either, all of them have monthly sale fliers with loss leaders and often have belts at prices so low it is hard to imagine wanting to design around anything other that what is available.
i think belts bought when prices are low, can be wrapped in paper and kept in a cool dry and dark place and probably last for decades.
bob g