Hmmm...this just shows the spectrum of how everyone finds Maths at AS. I personally am struggling quite a bit despite achieving an A* in Maths at GCSE level. I find the questions require a lot of logic to first unwrap what the question is really asking, apply the method, and then verify you have the correct answer. Since I'm doing normal Maths at AS (not F.Maths), we only do C1 in January which has actually got some tough bits and bobs regardless of how 'easy' everyone claims it to be.
I do think you'd be suited to do Maths over R.S. It will help you a lot by opening various doors to other course choices when the time comes for applying for uni. R.S is well respected too, especially being a humanist subject, however, I'd say Maths will prove to be more efficient and beneficial for your future.