JPG is smaller than PNG and opens everywhere. The difference between a good and a bad conversion is the quality setting — so here you set it, and you see the resulting file size before you download.
JPG throws away detail to save space, and the quality setting says how much. Below about 75% you start to see it in flat areas and along edges; between 85% and 95% a photo is usually indistinguishable from the original while still being far smaller than PNG. For screenshots and text, use a high setting or go to PNG instead — sharp edges are exactly where JPG struggles.
Is the file size fixed by a form or an upload limit? Use the version that targets a file size.