24g vs 26 g

Many of the systems made today are 26 gauge and they have good performance testing. That said, is heavier metal ultimately stronger and longer lasting? Yes. But some may say it's "overkill". I'd look at things like: 1) Warranty; 2) Installer Expertise; 3) Coating Quality; and 4) Any performance testing on the panel that they can provide. Installing over battens can be a way to smooth out the roof deck a bit which helps avoid oil canning. That said, it makes the roof harder to walk and can also make it less wind resistant. The air gap created by the battens though provides a thermal break to help with energy efficiency. For even better efficiency, cross battens can be used -- vertical battens and then horizontal. This allows for active ventilation beneath the panels. In any event, good attic ventilation is also helpful and lessens the need for venting and thermal break beneath the panels. I hope this helps. There are pluses and minuses to everything.

R Panel is a corrugated exposed fastener product. Unless it is forced to go over an extremely uneven surface, it usually does not show oil canning.