For most camera photos, JPG is the better target. PNG's lossless compression works brilliantly on screenshots, logos and flat graphics, but photos are full of noise and fine texture that it can't compress well. A 12-megapixel iPhone photo that becomes a few megabytes as a high-quality JPG will often be 10–20 MB as PNG.
PNG makes sense when the file is a working copy rather than the final result: you are going to edit it repeatedly, layer it in a design, extract a subject, or hand it to software that only accepts lossless input. Each JPG save adds a little more compression damage; PNG doesn't.
It is also the right choice for HEIC images that aren't really photos, such as screenshots or graphics saved as HEIC, or images with a transparent background. Converting those to JPG would add compression artifacts around text and edges, and fill transparency with white.
If you are simply sending photos to someone, uploading them to a website or printing them, choose HEIC to JPG instead. You'll get much smaller files with no visible difference.
Either way, colors are converted to sRGB. iPhone photos are captured in the wider Display P3 color space, so very saturated colors may look slightly less vivid after conversion.