Hey dude! I do FM, Maths, Phys, Chem, and CompSci self-studied in Y12.
From what I know, Chem is definitely the tougher A Level, out of the two, CompSci A Level provides a lot of breadth but less so on depth, and is good only if you're considering working in a similar or related degree path post-18, most of what you cover in CompSci A Level is taught mostly anyway, but is good if you really enjoy the subject.
Honestly, pick 1) what you enjoy more, what you're more passionate about, which links into 2) what you feel like you're more likely to succeed in.