I have both but it's difficult for me to compare them because I get a much better seal with the K171 than with the K271, so I think I'm not really hearing the K271 the way it's meant to be heard.
They're quite similar in overall style - neutral, clear, good highs - but the better-sealing K171 sounds noticeably more vivid and live to me. The K271's lows may go a bit deeper. Overall the K171 gets a lot more on-head time than the K271.
But again, I don't get a good seal with the K271 and your mileage may vary considerably.