get the 100mm, u wun go wrong.
just in case u decided to get a 82mm canon wide angle lens (16-35mm) in future. a lot of third party lens also using 82mm especially sigma lens.
85mm will be a tight fit if u have a 82mm lens later on.
nothing to do with cropping because your filter will cover the whole lens (front element).
personally i wun rely too much on m4/3 cameras for landscape shoot (to be frank). i find the models now (at least for now), still cannot match up to FF DSLR cams. Reason being, the widest length u can go is still 24mm (crop factor of 2 for 12mm) while a FF DSLR the widest lens u can use is sigma 12-24mm (widest you can go is a whooping 12mm!). This alone, m4/3 cannot match. Quality wise, subjective but IMO, still cannot match.