T40 and T50 are both office chairs with ergonomic adjustment functions.
However, the biggest different between each chair would be how the lumbar support lifts you. T40 has a more softer touch, allowing you adjust the height of the lumbar support. T50 supports you more firmly, allowing you adjust the height and depth of the lumbar support.
Also, T40 has easy adjustments as you can change the tilting degree with 3.5 turns of a lever, while T50 can be adjusted more subtly, with 23 turns of a lever.