ChipFlip
Two Zowie mice, $16.39 apart, and the difference between them is the shape of your hand. The S2 sits at $167.68 used, the ZA12 at $151.29. Zowie doesn't do features, so this comparison is unusually simple and unusually personal.
The S2 is the short, egg-shaped one
A safe hump, moderate length, and a profile that suits claw and relaxed palm grips. The S series is what Zowie recommends to people who don't already know what shape they want, and that recommendation is sound.
Medium hands tend to land on the S2 without much drama.
The ZA12 is taller and narrower
A higher back hump and a more pronounced rear flare, aimed at claw and fingertip grip. If you hold a mouse with your palm mostly off the shell, the ZA shape gives you something to brace against that the S doesn't.
That taller hump is exactly why some people can't use it. There's no middle ground with this shape.
Neither has software, and that's intentional
DPI, polling rate, and lift-off distance are all set with buttons on the underside. No drivers, no profiles, no cloud sync, nothing to update. Plug it into any machine and it behaves identically.
For competitive play that's a feature rather than a limitation. For anyone wanting per-game profiles or RGB, it means these are the wrong mice entirely.
Bottom line
This is a hand-shape decision, not a value one, and $16 shouldn't move it either direction. Claw or fingertip grip with the palm lifted points at the ZA12. Anything closer to a relaxed palm points at the S2. If you genuinely don't know, the S2 is the safer default.