Something else to throw into the mix...
As someone who applied for both DE and Uni (Aerospace at Hertfordshire), I opted for the degree when the decision came.
For me the decision was based mainly on the fact that I didn't feel ready to join the RAF. I'm by no means 'immature' nor anything of a 'mummy's boy' however I just felt within myself that I wasn't ready physically or mentally to leave home, finish education and join the Armed Forces. I didn't need to go to Cranwell only to be (more than likely) told 'come back in a few years wee laddie' - I already knew!
University (aswell as giving me a fall back for EngO and better opportunities later in life), also gave me 3 years to mature more, learn more, get fitter, and hence giving me a better grounding for OASC and IOT when they come. Ontop of that I get cheap beer, plenty of women, the opportunity to try for UAS/OTC, and give myself the best chance of succeeding at what I want to do.
If however i'd felt ready to join up at 18, I would have, it's just that my feelings (rightly or wrongly) were that I needed those 3 years to better myself before looking at trying for such a demanding and highly sought-after career in the RAF.
At the end of the day it's your decision, only you know what you want and how you feel, just think through what will give you personally the best chance of getting it.