My take on wildlife systems...
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Hello,
First off, to all that are looking to buy a new system I recommend this fun and informative reading:
https://theonlinephotographer.typepad.com/the_online_photographer/2010/05/letter-to-george.html
That out of the way, here is my take on your questions:
Is the E-M1 significantly better at photographing BIF? So much better that I should consider it over the G9 even if BIF is not my main concern but it's also not something I'm willing to neglect.
https://mirrorlesscomparison.com/best/mirrorless-cameras-for-birds-in-flight/#cameras
The E-M1 II is ahead by a somewhat serious margin. This translates in a somewhat better AF overall for the E-M1 II for all kind of wildlife scenarios.
Both are bottom of the pack though...
I haven't got any lenses at the moment, which lenses are the best between Pana and Oly? Also, is it worth waiting on the Oly 100-400 lens or is it already determined that the quality will be inferior to the Leica 100-400?
It depends on what and how you are shooting... I for one like the Panasonic 8-18 over the Olympus 7-14, the Olympus 12-40mm f2.8 over the Panasonic 12-35mm f2.8 and so on. The beauty of MFT is that you can mix and match high quality lenses from multiple providers without taking a hit on functionality.
Until a lens comes out, it is hard to tell how good it will be. My tea leaves tell me that the Oly 100-400 will be slightly better than the Panasonic 100-400mm. But I wouldn't make buying decisions based on tea leaves.
Which is the best system to begin with considering future expectations? I would really hate to invest in a either one of them only to read that they will abandon the format (I know no one knows the answer to this question, but some might have a more informed take on this than me).
Actually, the answer to this question is quite simple for a serious wildlife shooter (unless there is a medical reason that makes carrying anything over 2kg impossible): Nikon, with Canon a close second.
Reason for that is growth potential, both in the sense of lenses and sensors.
With MFT you are stuck with the sensor size (you'll get better sensors in time, but so will everybody else, and when it comes to sensors and wildlife, size does matter).
With MFT, for lenses, you either go for the 75-300/100-300 consumer tele lenses, which aren't really great, you sidestep into the 100-400mm (which in my experience with it, is a bit of a disappointment) or go for the optically superb, expensive and heavy (for the system) 300mm f4 and 200mm f2.8.
So, putting it all together: get either a Nikon D7500 or Z50 (if you prefer mirrorless) and the Nikon 200-500 if you shoot 60% wildlife and 20% BIFs. You can get it in budget if you shop around and it will out-wildlife and out-BIF anything in MFT land short of a E-M1x and Oly 300mm f4.
If you can't squeeze the 200-500 in the budget, go for the Sigma 150-600mm C or the Sigma 100-400mm C. Both are strong performers, above what is currently available from MFT tele zooms with similar reach.
With this set-up you'll have access to full frame sensors if you desire/need in the future and you'll be able to access various high end lenses (for example, the light 300mm and 500mm PF lenses or the optically superb, expensive and extremely heavy 400/500/600mms). Not to mention the vast used market and various either lenses.
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I hold the truth... A very specific, based on my experience and only relevant to me truth, but the truth nonetheless!