OK, here are my thoughts on the ZST and ZS3. In short, the 2 are both really good but have quite different sound signatures, so pick the one based on your music/sound preferences. My own preference is the ZST as I prefer more detail and resolution and I'm NOT all about that bass.
ZST
Very easy to get a good seal every time, and the bass is there though not overwhelming. Mids and highs are very clear, lots of detail and resolution. Medium sound stage, I've had a wider sound stage with my xiaomi pistons gen 5 (the new 2DD + BA hybrid) but that one also has a more airy feel, perhaps due to the front and back ports.
Silver vs Stock cable: as I wrote previously, there's a slight impedance difference between the 2 wires (1.2ohms vs 1.5-1.6). Not enough to matter in actual listening though, I have a feeling the silver wire makes the ZSTs a TOUCH brighter but that may be all in my head, as the wire swapping take a fair bit of time.
ZS3
ZS3 has oh so much more bass, almost like a wall of bass. Great for EDM and bass heavy music, but not as good for jazz/classical as the highs are a bit recessed and muddled. Mids I would say is a tie, ZS3 has warmth in the vocals, while ZST has more resolution. I also prefer the wires on the ZST that don't have the memory wire, which actually makes them more comfy and easier to store. (Did someone here say they took out the memory wire? Any instructions on how?)
Both are super comfy to wear, but I find I have to adjust my ZS3s more frequently than the ZSTs when I'm walking around. The litmus test is when I put in the earbuds and get a good seal, then make a big smile which reduces the seal coz my ears move, then go back to normal. ZSTs tend to go back to a good seal, while the ZS3s sometimes will stay loose and I need to adjust.
For extended/serious listening I used to use my UE TF 10s. They still sound better than the ZSTs (so much more resolution and richness) but unfortunately the comfort level is lacking...my ears tend to ache after an hour or so especially with the reverse mod, but then i also don't want to look like a teletubby. Anyway I think the ZSTs will be my daily drivers from now...
Bottom line, for the price both earbuds are a no brainer, pick the one based on your preferred music type and sound profile. If only their mic had volume controls as well then that would be perfect...at least for Android phones to keep the costs low. I also can't wait for the ZSRs to come out, if they can tune it well it should be really awesome!