Could anyone tell me what the magnification and the apparent horizontal field of view through the optical view finder is on the M240 (or M9)?
I believe the M9 and M240 both use .68x magnification in their finders. This makes the 28mm frame lines a little easier to see than on, say, an M4-P or M6 with .72x magnification, at the expense of a bit of size for 50-75-90-135 frame lines and focusing patch. But the total viewfinder FoV is still pretty close to the same if you look to the physical edges of the finder.
Personally, I like the M4-2 finder the most as it has .72x magnification with just 35-50-90-135 frame lines. I mostly use a 50mm lens on the M, this is the most comfortable and least busy finder to my eye.
For the M9, I bought a 1.25x auxiliary magnifier which I often use with 50-90-135mm lenses to enhance focusing accuracy and frame line comfort.