For some reason I've always assumed the older formats (Office 2007/XP) DOC/XLS were the safest to maintain compatibility with Office simply due to them being aged and likely more mature, however I've recently been told one of the reasons/benefits for DOCX/XLSX is that the old formats are actually an abomination. This has thrown my whole decision of which suite/format to focus on into disarray!
I hope this question isn't seen as breaking any of the SE rules in asking for a recommendation, I more looking for statistical or absolute proof of the best combination rather than personal preference.
Thanks!