The Stax is Ledger's premium device with a curved E Ink touchscreen. The Nano X is the practical workhorse. Both are excellent, but they serve different buyers.
The Stax's large E Ink display shows full transaction details and even custom artwork, making it far easier to review what you are signing. The Nano X has a small OLED screen.
The Stax supports wireless charging and long battery life, while the Nano X charges over USB-C. The Stax is more convenient for heavy mobile users.
The Stax costs roughly twice as much as the Nano X. That premium buys a better display and a more premium feel, not better security.
The Stax is for enthusiasts who want the best experience. The Nano X is the rational choice for most people.