>>2604608M4/3 user here.
/p/ talks shit about M4/3 because they think that the sensor is on par with cell phones, when in fact it is only about 1 1/3 - 1 2/3rds of a stop less than Sony's full frame sensors. Perhaps some of them feel threatened that their long term investment in Canikon DSLRs and lenses is losing relevancy as more and more people try to dump their DSLR collections as they switch to mirrorless, pushing down the price of used lenses.
As far as lens sharpness for price paid, M4/3 is unbeatable. Nearly every mid and high level lens is absurdly sharp, even wide open. Examples are the 25mm 1.8, 45mm 1.8, 12mm 2, 75mm 1.8, and all of the "PRO" lenses. The "PRO" zooms are a real value. the 12-40 2.8 is only $1000 new, and can be had for $600 used. Canon's is $1899. The 40-150 f/2.8 is a lens that cannot be matched. Does anyone else even make a 80-300mm f/2.8 lens? Anyway, its absurdly sharp. Just magnificent.
The E-M1 will give you better AF Tracking than the E-M5 II. If you are doing a lot of moving animals, it will be a bit better than the E-M5. For single point to point AF, the E-M5 II is just as good.
And yes, M4/3 is pretty much the perfect balance for light weight and top performance. An E-M1 or E-M5 II with the 40-150 f/2.8 and the 1.4x teleconverter would be incredible for wildlife.
For high ISO, even the smaller M4/3 sensor is so good these days that it really doesn't matter unless you want to shoot at f/8 in the dark, taking pictures of running animals. But even an A7S would have trouble with that.
I don't think you'd regret switching from Canikon DSLRs to M4/3, unless you are one of those people who LOVES shooting at f/1.2 on full frame.
