I would choose Delta over VS for coach trips because your status may mean something with Delta where it tends to mean nothing on VS. Plus you can pre-select seats in C+. VS has newer planes on this route, such as the A350-1000 (I came home on a VS 350-1000 in Y last March; the plane was over 50% empty and I had three seats to myself near the back of the plane (I think I was in row 70), so it turned out to be a comfortable flight.

For trips in Premium (PS) or D1/Upper Class, Virgin tends to have better food and beverage service plus a generally more enthusiastic FA crew. I've gotten to the point where I eat in the SkyClub on a USA-UK trip and sleep on the flight, skipping dinner and hopefully through breakfast (an eyemask is your friend).

Coming home from LHR, you don't get to use the Virgin Clubhouse as a PM or a DM unless you are flying Premium/PS or business. I think Delta has an arrangement with the "Number One Lounge" in LHR if you are a PM or DM traveling in Y; if you have an AMEX card, you can get into their Centurion Club (at the end of duty free in LHR T3), where they don't have windows but they do have an excellent "full English" breakfast buffet.