My background is a hobbyist with a very active and always running toddler (2.8 year old). In fact he never stops, he only stops when he goes to sleep. And when he stops for less than a second his head and body never sit still, he always moving. I've owned most cameras from most manufacturers through Canon 5DM3 on Canon side and through Nikon D800 on Nikon side. Also owned most mirrorless up to Oly EM1, Fuji XT1. As many others I am sure I got tired of carrying a DSLR with few lenses and a baby bag in another hand and dealing with a child while having large camera hanging on me. So Enter Sony. And I went all Sony.
I owned the original A7, A7r, and A7II from A7 class. I've never owned A7s. A7r - great camera but I could never get a still shot of my son. Too slow for that child. So I had high hopes for A7II. After a month and a half with A7II I sold it this morning. Nice camera but again the AF is just not there for my general use - It is clearly better than A7r and A7, and I was able to track my child running toward me outside however, inside in low light... not so awesome.
My other two Sony cameras are A6000 and A850. Ironically A850 with its ancient AF mechanism, I get much more keepers of my child indoors in focus with same lenses than A7II with either native FE lenses, or A7II with LA-EA4 adapter and same A mount lenses I use on A850 (24-70/2.8 ZA and 85/1.4 ZA). However, A850 produces great shots at base ISO, however high ISO is not a match for most others modern Full Frame Cameras.
So I sold A7II and I am thinking of these 5 routes:
1) A7s. Could it be a perfect camera for me? Low light higher shutter speeds and impressive low light performance? Is this camera specifically designed for me? but how is AF (I know EV-4 is great but how is the speed of acquisition?) It is nice to know that your camera will find the focus eventually in low light, but my child will be gone by then. So I need both great sensitivity and quick acquisition speeds.
2) A77II. Could it be a perfect camera for me? I have not tried it, but Gary Fong and some others state that despite the fact that it has less phase AF points than A6000 it is actually faster camera than A6000 due to 15 x-type phase af points and overall operations. And it has EV-2. (A6000 is EV0) Would it be my perfect camera? Will I like its high ISO results?
3) Wait a month to see if the next Sony A7 class camera is the holy grail.
4) Go back to Full frame DSLRS of Canikon - all problems above solved - great AF speed, great low light performance, many keepers, but then having to deal with original problem of having to carry large and heavy dslrs when I am not inside.
5) Go to m43. Quick acquisition speeds in low light, best Face recognition AF on the market, superior IBIS in Olympus, small and fast lenses. Problem: Every picture shot above ISO800 is disguasting with noise as a result of small sensor size.