What is the best way to save .docx?

Document with .docx extension can be saved as word 2007-2019 or office open xml. Wich way should I use to create document more compatile with word?

Hello,
from my point of view, there is no difference. Here is the result of a test file and the contents is almost the same.

user@system:> unzip -l file.OfficeOpen.docx
Archive:  file.OfficeOpen.docx
  Length      Date    Time    Name
---------  ---------- -----   ----
      573  2019-03-12 10:40   _rels/.rels
      541  2019-03-12 10:40   docProps/app.xml
      794  2019-03-12 10:40   docProps/core.xml
     1060  2019-03-12 10:40   word/_rels/document.xml.rels
    31021  2019-03-12 10:40   word/document.xml
      256  2019-03-12 10:40   word/media/image3.png
      653  2019-03-12 10:40   word/media/image1.png
      296  2019-03-12 10:40   word/media/image2.png
     8902  2019-03-12 10:40   word/styles.xml
     5135  2019-03-12 10:40   word/numbering.xml
      208  2019-03-12 10:40   word/settings.xml
     1396  2019-03-12 10:40   word/fontTable.xml
     1714  2019-03-12 10:40   [Content_Types].xml
---------                     -------
    52549                     13 files


user@system:> unzip -l file.Word2007.docx 
Archive:  file.Word2007.docx
  Length      Date    Time    Name
---------  ---------- -----   ----
      573  2019-03-12 10:39   _rels/.rels
      541  2019-03-12 10:39   docProps/app.xml
      794  2019-03-12 10:39   docProps/core.xml
     1060  2019-03-12 10:39   word/_rels/document.xml.rels
    31021  2019-03-12 10:39   word/document.xml
      256  2019-03-12 10:39   word/media/image3.png
      653  2019-03-12 10:39   word/media/image1.png
      296  2019-03-12 10:39   word/media/image2.png
     8902  2019-03-12 10:39   word/styles.xml
     5135  2019-03-12 10:39   word/numbering.xml
      208  2019-03-12 10:39   word/settings.xml
     1396  2019-03-12 10:39   word/fontTable.xml
     1714  2019-03-12 10:39   [Content_Types].xml
---------                     -------
    52549                     13 files

PS: I did not make further effort by comparing file by file within the documents.

Thank you!

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For better compatibility, in my experience, it is still better to save to DOC (Word 97-2003).

I found a similar topic: Why does Writer offer two options for saving in DOCX format?