Do what you want to do, both are respected A Levels so there isn't an issue there.....
Also be warned that most A Levels are different from GCSEs - Biology was my favourite subject at GCSE and now I'm not so keen... but I really love AS English Lit! It depends whether you like there being a definite right answer, or something that is more subjective!
As to what is in the English Lit course, it depends on what exam board it is etc
I'm doing AQA Lit B, and for that you have two pieces of coursework on plays, and then study two novels and two poets for a two hour long exam - I'd imagine most boards cover a similar kind of thing but I don't know...