weighing a sweet potato

It doesn't matter, they are both units of mass. For logging I'd say you'd have better results with grams however.

They do exactly the same thing, and there is literally no weight-loss advantage to either.

If you're unfamiliar with the units, the only thing to probably note is that "oz" are different from "fluid oz." The former is mass, the latter is volume (e.g. you'll eat 6 oz filet mignon (mass), but you'll drink a pint (16 fl oz) of beer). You'll find that 16 oz of water is not the same as 16 fluid oz of beer. This isn't really a concern because MFP uses all the right oz vs. fl oz measurements for solids and liquids. Just an FYI, really.

Grams are mass units (like ounces), but they have the advantage of being related to volume. 1 gram is the weight of one mililiter of water. It's really only an advantage to us nerds, who need to do those conversions at work

Lily is right though - there's no difference other than personal preference. Use whatever you're comfortable with. Personally, I use both. I have a food scale that does g and oz, and I wind up measuring them in whatever is used by the MFP data base . For example, I put spirulina algae in my morning smoothies. That I do in grams, because that's what someone already entered in the MFP database. When I measure chicken, I use oz because that's the food from the MFP database that's saved in my "frequent" list.

Good luck!