firstly a slight correction: VS
does have UCS (not the old j2000) to LAX
on its B744 flights as all LHR
B744's are now fitted with UCS. However if you fly on the VS23/24 this is normally an A343 which won't be fitted until next year.
If flying economy then it depends what your priorities are. VS
has good IFE and more often than not will offer better service (I know there are exceptions before I get roasted for that comment
does offer more room but again this will depend on when you are flying as all of AA
's aircraft are now having seats put back in the economy cabin.
Lastly do you have a preferrence of aircraft?. AA
will fly you on a B772, VS
on a B744 or A343. All good long haul aircraft but we all have favourites
So long 701, it was nice knowing you.