With the latest Defence Cuts announced recently, you do not have to be Brain of Britain to realise that competition is going to get harder in the near future (and stay that way). Therefore my gut feeling would be 'go for the degree'.
However, the RAF love a young recruit who will give them a minimum of 12-15 years front line service before joining the airlines. (New Terms of Service will be to age 40 to fit in with new pension scheme!!). Also I have met plenty of very sharp dudes who are moving up the ranks a lot quicker because they have age on their side rather than a degree. Universities accept mature students, the RAF don't.
So if promotion is your aim (which will take you out of the cockpit) then try and join early. If a good time and a cheap APTL by the age of 40 is your aim, then go to uni.
What would I tell my son to do? (another thread, I know). Well, if he wanted a career (ie high rank), I would first try and back both ways (being completely open about it.) ie "I would prefer to join the RAF first but if I fail I will go to Uni, join the UAS and tray again". At the same time, ensure he has a good uni place up his sleeve and if in doubt, go to Uni.
One of the standard questions at OASC is what will you do if you fail?. A good and realistic answer will paint a better picture than any bull about 'I only want to fly'. The RAF is looking for more than 'stick monkeys', it wants leaders who will achieve their aim despite the difficulties facing them.
Why do want to join the RAF??