I have the Olympus 12-100mm. It has more 'rotational play' on my Panansonic G7 and GX85 than my other lenses (more so on the G7 than the GX85). The 12-32mm Panasonic is particularly tight in comparison. I remember the same kind of 'play' when I had the 85mm Samyang lens so not just an Oly thing, but certainly a thing. It doesn't affect image quality, afaik.
I've had both lenses (currently have the 12-35 on my G85 and previously had the 12-40 on my EM1) and I can't tell any difference in IQ. They are both great. In reviews the 12-40 seems to have the edge at certain focal lengths but really there's not much it.
I tried both lenses a week or so ago when I bought the G85 and the 12-35 just sits on the camera better as it's a bit smaller ... feels right so I opted to buy it over the 12-40.
The 12-35 gives you dual IS as has been mentioned and works very nicely. No micro jitters at all. Probably not EM1ii stable (but nothing is) but very very stable indeed. Even the 35-100 @ 100 is super stable hand held for video using Dual IS.
The 12-40 focuses closer and is probably more rugged, however I remember a few of those falling apart too The 12-35 is rugged enough and splash proof anyway.
The extra 5mm doesn't really matter too much for me.
Both great lenses but I'd go for the 12-35 is using it on a G85.
Cheers.
