Using the highest quaility source (on iPhone) and cropping with PD will give you a bit more flexibility and potentially better results. Since during editing you can change the amount of cropping or undo and change (enhance) the source material, which cannot be done with the already cropped source material.
(It may also depend on the subject and the amount of cropping, the light available. What also comes into play is the quality of recording and required quality of final output.)
Best thing is to do a brief test with the two options to see what suits you best.

(And when you have done that, maybe you can share here, so that we all can learn?)

I have done both and although I haven't done a side to side comparison, there doesn't seem to be much of a difference in image quality. I tend to favor zooming in though since I can be sure that focus and exposure is calibrated accordingly. Downside is that these's less image stabilization compared to cropping where you can remove jitter.