So I had a few chances to audition the F, and Z, and think this thread has been very informative in outlining the slight differences between both devices, that is, sonically. Hence, the account below might be mere repetition... - I think I can now understand why someone alluded to the sound signature differences as being quite substantial relative to the actual technical competence of both. Though that was in reference to the F, and X, I think it's also the case with the F, and Z.
What was quickly apparent to me was the better treble extension on the F which provides a slightly more heightened sense of space. It's treble resolution seemed better, and micro detailing was a tad more perceptible to hear than in the Z. The overall sound feels light and cleaner than the Z, but I still don't quite hear the Z as lacking in details or clarity. It is just a thicker (richer), warmer, and slightly darker sound by comparison. The soundstage on both seem to be the same, with slight edge to the F in overall size, but I think it's just that the inherent richness of the Z gives the impression that details are being projected from a slightly closer range. With my IEMs the Z seemed more intimate.
I preferred the bass on the Z. The F's bass still seem to be a tad tighter, but on a whole. I'd say the Z sounds more authoritative and may have the better bass extension. It has, in my opinion a more tuneful bass that provides a good foundation to the music. To be frank, the F's bass is already sufficient and good enough for me, but I enjoyed the more weighty, and robust low end of the Z. Consequentially, I think vocals, particularly that of males, have more chest/heft on the Z., which I like, whilst the F comes across as a tad more articulate, and perhaps purer.
Overall, I found the Z to be more engaging. To my ears it ostensibly has the more coherent sound. The details it presents are more woven into the music, so it has a certain unity to it's presentation. When juxtaposed with the Z, the F came across as more analytical to me. Thus, as has been reiterated throughout the thread it really is a case of which sound presentation you prefer. My bias for coherency, bass, and subtlety in detail projection are the grounds for my preference for the Z. I just wish the Z was similar in size to the F or X. ...These impressions were taken with a Sennheiser IE8, in a noisy (ideal) environment!