Difference between spreadsheet formats that use ".xlsx"

In Calc (currently using 5.1.6), what is the difference between “Microsoft Excel spreadsheet 2007-2013” and “Office Open XML spreadsheet”? They both use the “.xlsx” extension.

And related to that: which one would be the “most compatible” with MS Office, i.e. the least likely to look different when sent to an Excel user, or the most “sustainable” solution (for data conservancy)?

As for interoperability with M$ Office, I strongly recommend using XLS rather than newer formats.

@gabix: how come? It seems counter-intuitive as XLSX has been “more documented” by Microsoft, right?

What makes you think that, as you state,XLSX has been “more documented” by Microsoft?

Because, contrary to the proprietary format XLS, XLSX is an open standard (part of OOXML), although I have heard that the documentation by Microsoft was poorly done (no ref), and the standardisation process was controversial (Standardization of Office Open XML - Wikipedia).
What I didn’t realise is that XLS, although being a proprietary binary, was eventually documented (Microsoft Excel - Wikipedia).
Was your first statement based on your experience?

Real XLSX/DOCX/PPTX files are not OOXML files, i. e. they follow Microsoft’s internal (and constantly changing) specs rather than the standard (the latter being born dead actually), which has been proved by independent researches. And yes, my first statement is based on my experience. Compatibility with the old XLS/DOC/PPT formats has been honed by StarOffice/OpenOffice developers for years.