this may be a somewhat annoying answer, but fwiw, I would caution you against asking the question in those terms. You should pick the IEMs that make the most sense for your personal taste, the music you listen to, whether they fit your ears (a real consideration with the IER Z1R according to many), price, etc; not based on a pairing with an amp. The W2 will work great with both—it's a super nice dac/amp. But it is by far the less important part of the audio chain compared to the transducers you are using.What would pair best with the W2 - Sony IER Z1R or UM MEST MK II?
(And while I would definitely not take my answer as any evidence about this as I am way less experienced than almost everyone else who might chime in, I think the answer to the actual question is that there is neither of really pair better with the w2 than the other, especially not in the absence of knowing more about what kind of sound you prefer.)