Gear choices

I don't own the Olympus 12-100mm. I had bought the excellent Olympus 12-40mm earlier before the 12-100mm was even announced, and it is my favorite lens. Of course the lens does not have the telephoto range or the IS in the lens that the 12-100mm has. On the other hand, it is a stop faster than the 12-100mm (f/2.8 vs. f/4) and I find myself often shooting at high ISO's (on 16MP bodies) that I feel I need that extra stop.

You mention viewfinder. My hot-button topic is the viewfinder. There are 3 types:

• OLED: Used in the G9 and Pen-F. Generally no issues with visibility, but I do tend to feel OLED viewfinders tend to saturate the image somewhat, and you have to mentally tone down the image somewhat to correlate with the image the viewfinder shows and what you will get. Some users are bothered by the lower refresh rate on typical OLED displays, but the Panasonic G9 claims to have a faster OLED display.
• TFT LCD: Used in the E-m1 mark II, and E-m1x (and the 16MP E-m5 mark I/II, E-m1 mark I). The biggest problem is if you wear polarized sunglasses, and shoot at the normal landscape orientation. Either the screen is completely opaque (E-m5 mark II) or parts of the screen are not viewable. I can generally frame the shot with the parts I can see in my E-m1 mark I, but I would not be able to manually focus. But typically the refresh rate is higher, and Olympus reps claim that is why Olympus has stuck with TFT LCD displays for their mid/high end cameras.
• Field Sequential. Used in the Panasonic GX cameras. I find these viewfinders to be too small to be useful. In addition, a significant percentage of people see a rainbow effect if the subject moves fast or the user moves his/her head fast.

In terms of lens IS and body IS interactions, you will only get a combined effect if you use an appropriate lens with a body made by the same manufacturer. I.e. Olympus 12-100mm and the E-m1 mark II, or the Panasonic G9 with the Panasonic 12-60mm f/2.8-4 or 12-35mm f/2.8 mark II lens. The 12-100mm will work with the G9 and the 12-35mm will work with the Olympus E-m1 mark II, but you will only get one IS system used, not a combination.

Speaking of the Panasonic 12-60mm there are two speed grades, the Panasonic-Leica 12-60mm f/2.8-4 and the Panasonic only 12-60mm f/3.5-5.6. Both are weather sealed, but if you are buying a lens, be sure you know which one you are getting. In the USA, Panasonic typically only bundles the 12-60mm f/2.8-4 with the G9, but it bundles the 12-60mm f/3.5-5.6 with the G85/G95. I don't own the 12-60mm f/2.8-4, but evidently it is only f/2.8 at 12mm, and it soon changes to a f/4 lens. I do own the Panasonic 12-60mm f/3.5-5.6 that I got with my G85 kit, but I don't use it as much in favor of the Olympus 12-40mm.

Note, it is a minor thing, but Olympus and Panasonic rotate their zoom/focus rings in opposite directions. Since I now own lenses from both, I just mentally have to switch gears when I switch lenses, but you may or may not want to consider buying lenses from just one manufacturer.

I've used the Olympus E-m5 mark I, Olympus E-m1 mark I, and the Panasonic G85 in wet conditions, including shooting on the boat ride at Niagara Falls where you get about 10 minutes of solid water coming down. My one incident was with the Panasonic 100-300mm mark II lens that got splashed by an ocean wave, and for a while did not work (evidently a salt grain got in the gears and jammed them until I gave the lens a sharp rap). My Olympus E-m1 mark I and Olympus 14-150mm mark II lens came through with flying colors (as did the G85 body that I had the 100-300mm attached to). Obviously salt is corrosive and be sure to clean cameras and lenses thoroughly with distilled as as soon as you can. It is best to avoid salt water if you can. But after 14 years of using various Olympus E-x and E-m1/5 cameras and lenses, I do tend to think they are truly splash proof (obviously carry something to clean off water off the lens).

For outdoor travels, I like to have a super-zoom (Olympus 14-150mm mark II) available so I can minimize lens changes. Panasonic recently updated their 14-140mm superzoom to make it splash proof now. If you are buying the 14-150mm used be aware that the mark I version was not splash proof and the mark II version is. For Panasonic the new 14-140mm was just announced, so make sure you are getting the mark II version. Olympus also has a new 12-200mm lens if you need both wider and longer focal lengths. It is somewhat expensive compared to the 14-150mm and 14-140mm lenses, so be sure you need the range before going with it.

Note some cameras crop when shooting 4K and only use the center part of the sensor (on a 20MP camera, this would be a 1.26 crop, on a 16MP camera, this would be a 1.18 crop). The G9 does not crop, the G85/G95 do crop. I don't know if the E-m1 mark II crops or not.

Both the G9 and E-m1 mark II are limited to 30 minutes of recording in software. There are evidently ways of bypassing this limit if you need to record one longer continuous segment. The G95 does not have a limit (nor does the non-European version G85 -- but the G80/G81 do have the 30 minute limit).

For JPG shooting, I find Olympus cameras don't burn out highlights in white areas as often as Panasonic cameras do, so I have to use exposure compensation less frequently. This makes it easier to capture decisive moment type shots easier if you have the camera at your side, and only bring it up to your eye to shoot, and you don't have to adjust things.

For JPG shooting, on Olympus cameras can be programmed to have a button add a toggle for 2x electronic zoom.  I find in practice that the Olympus 2x zoom works well, but the Panasonic version (2x and 4x) has more image degradation.  If you shoot RAW, you can crop to the center to get the same effect, but I find it is simpler if I have the button and I can properly frame the shot.

I don't have the G9 which is supposed to have improvements, but generally I find the Olympus JPG high ISO handling to be better than Panasonics. If you are shooting RAW, it doesn't matter as much, since you typically do the noise handling in the RAW converter.

If you are planning to shoot some movies, you probably should look for bodies that have at least a microphone jack.  Some bodies also have a headphone jack so you can monitor the sound being recorded.

While some improvements have been made in the G9, neither brand is known for their fast auto focus when shooting movies compared to other brands.  In general, I tend to keep the camera in S-AF mode, and just use the touch screen to select the focus point.

If you need to shoot for long periods of time, Panasonic cameras are easier to add dummy batteries that connect to a larger external battery or A/C power connector.  There are ways to do it on the Olympus, but Panasonic is simpler.