How about you do both of them and drop one in the first month? Everyones saying maths is the easiest, and I'd agree, but only for someone whos good at it. Thats cause you don't need to spend ages learning facts and key words like you do in other sciences. You just learn the methods and use your head.
Maths is very very hard if your no good at it. Very easy if you are.
Chemistry is hard whether your good at it or not, but easier if you are.
I honestly don't think you should do either if you get a B, but if your adamant about trying them, go for it! Take 5 A levels instead of 4 - a few of us did that in my school, me included. I dropped one after a month, cause I couldn't be arsed lol.
Give them both a go!