I'd back England. With Shilton (or Banks) in goal, Rio and Moore at centreback, and midfield of Edwards, Robson, Charlton, they'd have a colossal spine. Scotland's strikers (Law, Dalglish) might be better than England's.
Then again an all time Englan XI with the likes of Banks, Moore, Charlton, Greaves, Edwards, Hoddle, Robson, Shearer, Linekar, Gascoigne, Beckham, Barnes and so on would give 'em a game and a half.
For Scotland, how bout McCoist, and Colin Hendry? Both great players in their day.