I assume it's the same for everywhere that has a Virgin clubhouse, but the food at all clubhouses I've been to (LAX, SFO, EWR, JFK, LHR) is fantastic. You order whatever you want off a menu. You're not dealing with a standard buffet. So, if you eat at the clubhouse beforehand, you needn't worry about the main meal onboard, which I always find annoying anyway since service takes so long. Plus, the clubhouses are usually quite nicer than the Sky Lounges.
I haven't eaten on a Virgin flight in sometime, so I can't comment on the current state of the food, but I can say that Delta One food is usually quite forgettable unless you get one of the special menus out of LAX or JFK. The Jon and Vinny meatballs out of LAX, in particular, are great.
If you're booking with Virgin directly, at no extra cost, Virgin will arrange transportation to and from the airport. At LHR, Upper Class has a private airport entrance that leads to private security.
Virgin's seat has fairly limited recline (far more than a standard seat, but it can only be adjusted so far). So, if you plan on being awake most of the flight, Delta is more comfortable. Delta, while not offering much storage, still offers far more than Virgin. Virgin basically gives you a tiny shelf. I don't find Virgin's storage to be a problem when traveling lightly with an iPad only, but if you're working on a computer, it's going to be an issue.
When it comes time to sleep, I prefer Virgin over Delta. Yes, you're required to flip over your seat on Virgin (not difficult), but once laying down, the bed is plenty comfortable, and I find the open footwell to be much more comfortable than Delta's too-small closed footwells (unless you can snag a bulkhead seat).
When booking on Virgin, for more privacy, you'll want to book on the row of seats that faces the wall rather than the seats that face each other.