E-M1 II has nicer jpgs but G9 slightly better at high ISO
E-M1 II has nicer 4K video thanks to 24p DCI cinematic but G9 has 4K 60p not just 30p, and 1080 180p not just 60p
G9 has animal face detect AF not just human face detect AF
G9 has better buffer for burst mode - it can do over 600 frames at 9fps mechanical shutter
E-M1 II's ProCapture mode requires an Olympus Micro Four Thirds lens and allows 14 frames captured prior to shutter release, G9 captures previous 0.4 sec which gives 8 or 24 shots depending upon burst rate; G9 has a slightly faster burst rate.
E-M1 II has better C-AF for casual use thanks to having PDAF and unlike the G9's DFD CDAF, it works with any Micro Four Thirds lens (DFD only works on Panasonic lenses and gives a distracting effect in C-AF although may be more accurate), but the G9's CDAF single AF appears even faster than the super fast E-M1 II
both cameras have industry leading image stabilization but the G9 seemed slightly better - but for Dual IS you need Panasonic lenses for the G9 and Olympus lenses for the E-M1 II
G9 has a much better EVF (but other reviews have commented on the very annoying pincushion appearance)
G9 is larger, heavier but does have the top LCD display which can be useful, and a easier menu system
E-M1 II has some unique features such as in-camera focus range settings, preset MF distances, Night photography modes such as Live Composite, Live Timed, Live Boost
G9 has a unique night mode for astrophotographers which restricts displays to red to conserve night vision, and can be charged via USB