Is there a difference between .doc and .docx, and which should I upload to KDP Print?

There is a difference between .doc and .docx files, but it is mostly a behind-the-scenes technical one that really isn’t too important for the end user. For compatibility with collaborators (other writers, editors, etc.) you should try to save your files in the more modern .docx format when given a choice.

For uploading to KDP Print, I’d always provide print-ready PDF files and would not try uploading any kind of Word Documents; if you can avoid the printer from having to make decisions about how to print your book, you will be better off.