As to which is most important, depends on whether by the end of the season we have the CL.
The PL is won, barring an apocalypse.
The CL is an ongoing lottery. (I might have to have an excessive amount of beer to keep calm to be able to watch, if we reach the final).
If we play mostly 2nd or even 3rd string against United, I’d be meh about any potential loss - it would be bad for the very short term until the PL trophy was lifted, but in the context of the entire season, it’s only the ‘we beat city’ cup.
Along with the CL, if we also got the 100+ points, I’d say we could be reasonably happy.
So... Liverpoolx2 and all the other CL games are the most important.
The United game is unbelievably (at the start of the season) not so much, barring bragging rights imho.