They use the same sensor but a different radio chipset - T1 uses Zigbee 3.0 and therefore requires a Zigbee 3.0 hub, while the other uses Zigbee 1.2 HA and will work with any hub. Both offer the same functions and performance with no special benefits (i.e. no "longer battery life" or "longer range" nonsense).

Old versions are still sold to support the larger customer base with older hubs, and they are often cheaper too.