Originally Posted by
GrumpyDiver
On which camera body? Olympus goes purely for in body stabilization, so either lens will work on the body, but the Olympus lenses do not have in-lens stabilization. Panasonic tends to go for in lens stabilization and some of their more recent bodies do both (even at the same time). If you go for a Panny body, Panasonic lenses are a better option. From what I can see on the Panasonic website, there are no plans to upgrade the in-lens firmware 100 - 300mm lens so that it can use both in lens and in body stabilization at the same time.
From a pure lens performance standpoint, lenses that have a lower zoom ratio tend to be stronger performers from an optical standpoint as the lens designer has fewer constraints to worry about. If you look at "pro" lenses, they rarely exceed a 3x zoom capability, purely for this reason. Based on this, I would expect the Panasonic lens to be a slightly stronger performer.
As a general rule, I find that while both Olympus and Panasonic are technically interchangeable, what Robin points out is correct. The mFT cameras write lens characteristics into the metadata so that both the jpegs AND the raw files are impacted by the lens correction data. I would tend to recommend using Olympus lenses on Olympus bodies and Panasonic lenses on Panasonic bodies for that reason.