Hey, I was in the same position as you last year and you actually did better than me in your AS levels. I got AAAB (the B in Eng Lit!) with a 92% average, and now I'm going to Cambridge in October to read English. I would base it off your preferences for the course and ignore what could be 'easier', as you know both are difficult to get into. I decided to apply for Cambridge since I loved the breadth/depth of the course and was put off by the Oxford modules. Also, my GCSEs were 3 A*s, 3 As and 5 Bs, and knew that despite not demonstrating my potential in the ELAT, I could at interview - most Camb colleges require you to sit a written test (which isn't as scary as sounds - I was terrified and actually enjoyed it!)
It is a fact, certainly it seems more with Arts courses, that Cambridge interview more people than Oxford do - my hopes hinged on being able to prove myself despite my B/ 'average' GCSEs through the interview; plus most colleges at Cambridge (I don't know about Oxford) want to see two pieces of written work as part of deciding who to interview, which is great because you can expand on them in interview if you are invited.
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Everyone with a realistic chance of being offered a place is invited to attend an interview. That’s more than 80 per cent of applicants each year.'
http://www.undergraduate.study.cam.ac.uk/applying.
Your UMS averages are highly likely to demonstrate to the admissions tutors that you have potential - if I can get an interview with a B in the subject I was applying for, then you're arguably in a stronger position!
I know from personal experience of my sixth form that Oxford seem initially very strict on who receives an interview, whereas Cambridge reject a lot of people after them - it does seem that Cambridge give candidates with potential the chance to prove it - I can't comment fairly about Oxford as this is just my experience.
PM me if you want to ask anything, from personal statement/interview tips/analysing texts
I know applying feels like a risk, but you could regret it for the rest of your life if you don't take it.