The Short Version
After 60 days of testing, the WHOOP MG (26.5g, screenless, 10-12 day battery) proved better for passive health tracking, while the Apple Watch Ultra 2 suited users wanting apps and notifications.
After 60 days on the same wrist, the question was never accuracy. WHOOP has no screen, no buttons, and a charger that clips onto the band so you never remove it. Apple Watch is a small computer you happen to wear. One device wants to disappear. The other wants your attention.