The Flex and Stax are both modern Ledgers with E Ink displays and NFC. The differences are mostly about size, shape and price.
The Stax has a curved, credit-card-like form with a full E Ink screen. The Flex has a more traditional flat layout with a slightly smaller display and a keyboard-style case option.
Both support NFC, Bluetooth, wireless charging and a huge range of assets. They share the same secure element and security model.
The Flex sits below the Stax in price, making it a premium device at a slightly more accessible cost.
Choose the Stax for the flagship experience. Choose the Flex if you want most of the premium features for less money.