Virgin Atlantic or Delta? (Flying LHR to SFO)

[R.O.S.S.I]

Hi all, I am being flown out to San Fran for a work meeting in a couple of weeks and have been told to book some flights over there.
Much to my dismay they wont pay for business but have said they will reimburse Premium Economy tickets.

Looking at timings and costs the only two options are Virgin Atlantic (787-9) or Delta (777-200 and 787-9).

Being that I haven't flown with either airline before and that I've only really ever flown cattle class before I was wondering if any of you fine folk have opinions on which airline is better overall?
In particular which would be a better Premium Economy experience given they both offer one (especially on the LHR to SFO route)?

 

[R.O.S.S.I]

I know you must all be waiting with bated breath but I ended up booking Premium Plus with United as the flight times and cost worked best.
Flying on a 777-200 on the way out and a 787-9 on the way back.