Convert PNG to JPG to save drive space -- non-lossy

I've been trying various combinations of ffmpeg and imagemagick to find the optimal parameters for converting PNG to JPG. I'm trying to save drive space and would like to retain as much image quality as possible.

I generally find even default parameters for these two terminal commands produce a good result. However, since I am trying to save drive space and plan to delete the PNGs after conversion, I'd like to know that I'm using the best possible command to accomplish it.

I'll be running it in batch, probably using find -exec.

Thanks