Seek clarification of XMP synch options

As I complete my move away from Aperture I find I am confused by the consequences of the XMP synch options chosen.
The choices for auto synch with sidecar amp are full synch, none, and load. There is also the independent option of choosing prefer embedded xmp over embedded IPTC and Prefer sidecar XMP over embedded metadata.
My goal is to be able to do keywording in both C1 and Photomechanic and perhaps a Digital Asset manager such as Media Pro or Photo supreme. I also use geotag to add lat long data.
In playing with the options I find sometimes star rating can be lost as well as keywording (this happened when I choose load as the auto synch option in capture One.)
I know that Photomechanic and Lightroom will both use xmp sidecar files. It would seem that C! keeps its data within the catalog (I am using a catalog.)
When I lost the keywords and image rating I had neither prefer embedded or prefer sidecar checked.
I guess what I would like to know is where C1 stores its metadata - perhaps different with different options, and which choice of options is least likely to result in metadata such as rating and keyword loss.
My guess is that with load selected as auto synch option it will overwrite anything generated in C1. Perhaps checking one of the prefer options would make a difference and perhaps Checking full synch rather than load would make a difference. Or is it best to do no auto synch?
Thanks for any insights.