Actually LUVOIR will most likely have smaller 6 meter mirror to reduce cost and use lessons from JWST construction. It will be simpler design than JWST as it will use only 2 or 3 layer sunshield and no cryocooler (creating equipment that can work at 5 Kelvin was massive issue for JWST). Its main science goal will be examining atmospheres of exoplanets, plus general observations like Hubble.

It will observe in UV/visible/near-IR so its resolution will be higher than JWST's as shorter wavelengths are easier to focus (diffraction limit is a function of wavelength) but it will not be able to see objects at the very end of observable universe as it cannot image in mid-IR or longer wavelengths. There is still plenty of interesting stuff closer to us and it will be "true" successor to Hubble.