Edexcel chemistry and biology AS/ALEVEL

should I take AS chemistry or take A2 biology instead?which is easier to guarantee higher grades?

Oooff hard choice. I am doing both Chemistry and Biology as alevel. And both doesn't get easier for A2. Biology A2 gets alot harder this is because paper 3 is essay and there's more content to learn than AS so you have loads of cycles to learn like nutrient, nitrogen, Calvin , krebs. And more things connecting to ecosystems, genetics etc. However AS chemistry is more easier since there is less content but it's got multiple choice questions which gives you limited time. And really the choice is all yours. I am just informing you on the difficulties. Hope everything turns out well for you.