While the Apex Pro TKL features SteelSeries' patented OmniPoint 2.0 switches, the Apex 9 series include next-gen OptiPoint optical switches. OptiPoint switches use optical light sensors for a faster, more responsive, and more consistent input than traditional mechanical switches.
Also, unlike the Apex Pro TKL switches, the Apex 9 series keyboards have fully swappable switches for customization and longevity.
OptiPoint switch:
OmniPoint 3.0: