The James Webb Space Telescope has a primary mirror of 6.5 meters (21.3255 feet)in diameter. The Hubble has a primary mirror of 2.4 meters (7.87402 feet) in diameter. Think of the detail the James Webb will have with its 6.25 times more collecting area! Also, the James Webb has tremendous infra red capabilities (from 0.6 to 28 micrometers) while the Hubble studies objects in the ultraviolet spectrum. As the James Webb studies further into the cosmos, light shifts into the infrared spectrum.
20 odd years in the making too