I'm adding coordinates, location, country name, my name, url, and occasionally a description to my photos. They are all in the JPEG format. I use them mostly for personal use but give some away to others or upload to the internet. I do not sell these images. I prefer using open source software running on Linux and currently using exiftool to add most of the data. File size is a reasonable issue to me, but I prefer the best quality and compatibility I can get. I don't like the idea of having the same data multiple times in the pictures (for size matters but more because thy could get out of sync).
Whats is the best place to store it? Should I include a IPTC header or only use XMP? And should I add the same data as well to EXIF? For some of the pictures I have only the year or month and year and not a full date. I know XMP supports that but what is the best way in others?