a battle to the death… ok maybe not… ;) …just a comparison for those who want to know…
the M40s are always on this side < -----> the M43s are on this one. :)

the appearance: texture
the M40 yarn is more nubbly in texture, a slight bit thicker and the sock has a more open weave than the new M43s. the M43s’ knit is smoother and ‘flatter’ looking and the yarn is a bit finer and quite smooth, these would probably give a bit less indentation on the bottom of the feet for those who find that uncomfortable. The M43s do pill a little in high wear areas of the sock (the sides of the feet and the back of the heel, mostly from my flannel sheet lol) but I don’t think quite as much as the M40s.

the heel:
smaller foot M40s and M43s. I usually wear size 8-9 shoes.

the appearance: bagginess
M40s are baggy and wrinkle at the knees and ankles and tend to slowly slink off of my feet when I’m not wearing shoes, M43s snap right back smooth against the skin and they don’t slip off the end of my toes. yay!

They both go up to the top of the thigh on me the M43 was a little longer so I had to fold it a teeny bit. My leg length from the bottom of my foot to my crotch is a little more than 33 inches and the widest part of my thigh measures 21.5 inches for those who might like to know. ;) ;P