I think a preference in gimbal joint designs, clamp pressure and pressure points, ear pad preferences, and overall fit-and-finish will also make a difference between choosing electrostats.
I have made these smaller, but long-term significant, differences between the Stax SR-009 and a MrSpeakers Prototype Ether ES 5/2017 model for at least 2 sessions (at least one hour each) for each one:
- The SR-009 has more spherically-adapted gimbal joints, whereas the Ether ES 5/2017 has a more parallel-angled gimbal joint design.
- The SR-009 has overall slightly more clamp pressure than the Ether ES 5/2017, possibly from a more suspended headband design.
- The SR-009 ear pads have that infamous "Stax fart" (audible and sharp sound from foam compression/decompression), whereas the Ether ES 5/2017 ear pads have very slightly more stiff foam, making the foam "fart" less audible, though the Ether ES ear pad leather is very slightly stiffer in texture than the SR-009 ear pad leather texture.
- The SR-009 frame and headband materials feel more solid and luxurious than the Ether ES 5/2017 materials, but the overall SR-009 weight is heavier as a result.
I rank the importance for each listed point from least-to-most: 1, 3, 4, 2. I did this ranking as a single-line as this is my opinion, as I prefer the Ether ES 5/2017 over the SR-009 on this order of importance and leaving the list for others for however they want to look at my observations. The sound signatures between the two electrostats will vary greatly by what source, DAC, amp, etc. used, so I feel a sound preference would be redundant at this point. The gimbal joints between the two are very similar with the headband-to-gimbal angling being slightly different, but the other points will start showing their differences. The "Stax fart" from the SR-009 ear pads is somewhat distracting during fitting, but the softer leather texture kind-of make the fitting worth it, though jaw movements and neck movements may reproduce the "Stax fart" for a split-second. The SR-009 frame and headband materials do make for a more solid and luxurious feel, but having it on my head for more than an hour starts to show its cost in added weight with slight neck strain (not sure how it is for weeks, months, or years). With the added weight in mind from the more solid and luxurious materials, the slight clamp pressure will get slightly worse over time and may start to take your attention away from the sound signature.
With this said, I prefer the Ether ES 5/2017 with its lighter weight, more flexible headband, much less ear pad foam sound interference, and more-accommodating gimbal joint design for my head. This leaves me to listen and do whatever else, like typing. Hope this helps.