The main difference I can see is that insync creates MS file formats when it syncs (.xls, .doc etc) whilst GDrive creates .gsheet, .gdoc files which for me just open Google Docs in Chrome.

Interesting difference. Google's approach isn't very good for interoperability.

edit: and incidentally insync has failed to comprehend me moving a stack of files into a folder and instead created duplicates. Syncing is a hard problem with lots of edge cases (and on that note Chrome bookmark sync deleted 5 years of bookmarks)