I already did a specs comparison between the GFX100S and Sony a1 here based on the info available at the time. Now Gordon Laing has released a hands-on comparison of the two cameras, which you can watch above or read the summary of below:
4th GFX camera
Almost half the price of the GFX100 with essentially the same capabilities
Fujifilm GFX 50S is discontinued
The Full Frame Sony A1 actually costs more than the GFX100S
The big bump from the GFX50S is gone
Similar feels similar to a 5D or D800
Changing the battery let Fujifilm to move the battery into the grip
There is no portrait grip or dual batteries but it is smaller all around and 500g lighter
New battery can do about 460 frames so battery life is good but you will want a second battery
The EVF is now fixed and smaller than the GFX100, but it is still comparable to other cameras
Sony has a more detailed viewfinder, but Fujifilm’s viewfinder doesn’t hold the camera back
The main monitor hasn’t changed on the GFX
PASM mode dial on the GFX100S which is different than other GFX cameras
Can shoot for up to 60 min on the GFX100S
1.8″ eink display that’s like the one on the GFX100 and it can be reversed from white to black
GFX100S misses the second einck display of the GFX100
GFX100S should have larger controls that are backlit
GFX cameras feel like X cameras in the menus
UHS-II dual card slots that are fast enough for the 5fps of the GFX
12-bit RAW can be outputted via mini HDMI
GFX lenses are great, but there are more lenses for full-frame bodies
The 102MP sensor is the selling point of the GFX
The IBIS is very useful
Could shoot the 45mm f/2.8 with IBIS at 1/10th a second, but without he needs to shoot 1/80th
IBIS is also good for video
The AF is as fast as other cameras with smaller lenses
The GFX feels like using a smaller format camera
The standard test chart had to be shot at double the distance since the camera out resolved the test chart filling the frame
GFX100S easily out resolves the Sony Alpha 1
Both cameras do better with pixel shift and the Alpha 1 can out resolve a single shot of the GFX100S, but if you use multishot with the GFX100S you end up out resolving the Sony and chart again
No camera under 10k out resolves the GFX100
GFX100 can easily recover color data in the dark by lifting the shadows
4k recording is different on each camera due to how the cameras come up with their 4k from their sensors
Motion tracking in video is fine for somewhat static subjects, but the motion isn’t tracked well in video
GFX100S is Fujifilm’s most compelling camera to date
The GFX100 and GFX100S perform like smaller sensor cameras, but the GFX100S is more portable
The GFX100 can be shot casually thanks to its IBIS
If you’re into fast sports and wildlife a camera like the Sony A1 might be better
GFX only has 12 lenses at this time, but full-frame cameras have numerous choices
GFX100S is best for photographers that demand the best quality and it speaks to your heart in a way other cameras do not
There will be more videos comparing these cameras soon