I'm sure I read in a review on FM that the FE mount version of the 40mm f1.2 is better on Sony, here on page 40...
https://www.fredmiranda.com/forum/topic/1512530/39
If that's true I can think of one reason to still buy the M mount version, it's more portable as you can use it on just about anything but the Sony mount version is limited to, well, Sony I suppose.
More on 42...
https://www.fredmiranda.com/forum/topic/1512530/41
I've had the Sony mount version since it first came out but tbh I prefer the 35mm f1.4 which is a bit wild at f1.4 but from f2 or so and onwards it's a different beast and it's smaller and lighter but if your heart is set on the 40mm f1.2 I'd go for the FE, in fact I did. Even if they're the same, which they may not be, with the FE you gain exif and the ability to have the magnified view automatically magnify when you turn the focus ring also I don't know if the M mount one has the de click feature but the FE one does but personally I prefer the clicky setting.
I've been thinking about buying the 50mm f1.2 or maybe the new 50mm f2.