For mixing only, I'd go with the K240S.
If I'd use them for tracking, then I'd seriously consider the K271, since they are closed.
I have 2 K240S's and have been very happy with them. They might be a bit hyped compared to other brands/makes, but you just have to get to know them. They're comfortable to wear for long periods and that was a big deciding factor for me.
K271 do sound different, due to their construction, but I'd still be happy using them.