PNG stores transparency
PNG supports an alpha channel, which records how opaque each pixel is. It is commonly used for cutouts, logos, and interface graphics. PNG files may be larger than a compressed JPEG, especially for photographic images.
JPEG is opaque
JPEG does not store transparency. If you save a cutout as JPEG, transparent pixels must be filled with a solid color, often white. JPEG can be a good choice for ordinary photos where a solid background is expected.
Pick based on the next step
Keep a transparent PNG as your reusable cutout, then export a JPEG after placing it over the final background if a workflow requires JPEG.