I love Virgin and on the whole it's a far better airline than Delta. I fly JFK to LHR quite frequently and have flown Delta's A330-900neo and B767s many times. I've also flown all of Virgin's aircraft on the route including the Airbus A350-1000 and Airbus A330-900neo. If you are looking for the best food and service, I'd choose Virgin, but it seems you are rightfully prioritizing sleep which adds a bit more nuance. All of the above posts from other commenters are very fair, but I wanted to note two things from my experience.

1. I've found the Delta One Suites on the Airbus A339 to be longer than Virgin's beds on the A350-1000. The difference is about an inch or two, and this is confirmed on their websites. I'm 6'3'' and every extra bit of bed-length is helpful. Delta has a slight edge here.
2. Virgin's A350-1000 Upper Class suites are not true suites with closing doors. The doors are a side panel that slides out a few inches. I know there are varying perspectives on whether doors are gimmicks, but I do find them helpful in getting a better night's sleep. Delta also has a slight edge here.

As everyone else said, Virgin is overall a superior airline and a better experience, just thought I'd note those two points.