There are multiple ways to increase depth of field (DoF) in macro photography. I know about focus stacking, but I have a question about which way is best for a single-shot. Two common options are:
Reduce aperture. The depth of field is essentially proportional to the F-number. This trades image quality for DoF because of diffraction.
Increase the distance to the subject, and then crop the image (i.e. zoom digitally to compensate the distance to subject). This also obviously trades image quality for DoF (less pixels, defects of the picture amplified by cropping if displayed/printed at the same size as the non-cropped image). This is more or less the same as taking the picture with a camera having a smaller sensor.
My question is: in which condition should I favor one or the other? In other words: at constant DoF, how does image quality vary if I change the distance to subject and aperture? Or, equivalently, at constant optical resolution (i.e. constant size of the airy disk relative to the size of the picture) changing distance and f-number, where will I get the maximal DoF?