I have and regularly use both for work (events) and for fun (nature/landscape/tourism). The G9 is a better camera overall. For me this mainly means it has:
- Better AF, including better subject detection
- Slightly better IBIS
- Dual SD
- Stated freeze resistance
- High res shot
Plus much better video specs, joystick and more buttons overall. The top LCD is helpful at times as well. There really is a tonne of small features that give the G9 an edge.
The G90, however, has a few small advantages/aspects compared to the G9 that make it my favourite Lumix camera:
- Classic Lumix controls (on-off switch, front dial around the shutter button)
- Pressing the shutter button is a lot more tactile
- Its size and shape feel almost perfect for my hands
- Quiet mechanical shutter
- On-board fill flash
- EVF sensor on top, not bottom
- "Fun" filters, plus Live View Composite
It also has unlimited video recording (a necessity for some of my work) and it's noticeably lighter.
In general though given the small price difference I would suggest going to for the G90 only if you
really like some of those few things it does better/different than the G9.
Edit: I notice you have a G100, right? The G90 is basically a full fat G100 - with a different body, IBIS, weather sealing, and a proper mechanical shutter whereas the G9 is a different camera.
This is a superb description and covers a ton of the highlights.
As a fairly long-time G95 owner, I want to add a couple more things:
- In-camera sweep panorama mode (G95 has, G9 does not)
- 6k vs. 4k photo features (G9 has 18mp stills, G95 has only 8mp stills)
- Full-size RAW/JPEG/RAW+JPEG pre-burst/pro-capture burst capability: G9 has, G95 does not
- Battery life rating about 20% better with G9 vs. G95
- Larger EVF on G9 vs G95 (but G95 is still pretty decent)
- G95 will make "fake 12 bit" RAWs in certain high speed operating modes without telling you (e.g., if you do bracketing or in certain high speed burst modes at high shutter speeds). This is incredibly annoying because the camera doesn't signal/indicate when this is happening and when it isn't so you can decide whether you want to exit bracking, reduce your burst rate, slow down your shutter speed, etc.
What I mean is you'll get ordinary looking RAW files but they're actually 10-bit RAWs - only every fourth luminance value is used within the 12 bit container, so 3 out of every 4 luminance values is completely empty.
This lower bit depth translates into more posterization and much clumpier and uglier noise patterns in the shadows if you need to push and/or do noise reduction. This may not matter in a lot of cases (e.g, uncropped full sized JPEGs). But if you need to crop or pixel peep or push the RAWs, you'll definitely notice the difference. This hasn't been a huge issue for me, but on a couple of very rare occasions, it's been rather annoying. Not enough for me to get rid of the camera because high speed operations (bursts/bracketing) is such a tiny part of my overall shooting, but if I did, I would 100% choose the G9. (BTW: Even the lowly G100 was improved upon the G95 and doesn't exhibit this same behavior).
- Faster flash sync with G9 (1/250 vs 1/200)
- G95 has unlimited video recording but G9 has way more video recording options, including no crop 4k options (G95 has a 1.2 crop in its only 4k mode with which is 4k30 4:2:0 8-bit) and has true 10-bit Vlog and higher frame rate options such as 4k60 and 1080p180 with autofocus (G95's slow-mo 1080p60 is manual focus only, with an enormous 2.7x crop). The G95 1080p 8-bit video quality IMO is really excellent but you get more with the G9 by a wide margin. If you don't need unlimited video recording and want a more fully spec'd video shooter, I think you should opt for the G9.
OK, all of this is making me now giving me major G9 GAS! Why did you do this to me?
(PS: I like the G95 just fine. Very solid camera overall, especially if you just shoot stills. But I really hate that I can't have everything the G95 can do in the G9 as well... Super annoying to have the "consumer convenience/novelty features" only in the G95 [sweep panorama, live composite, onboard flash] while the "pro level" G9 doesn't have them because "it's a serious camera." :-x).