In AS i got a B (210/300, C1=74 C2=76 M1=60) and retook C1 C2 and M1 in my second year and got 284/300 C1=93 C2=95 M1=98. In my second year i got an A across C3 C4 and M2 and got an A overall. It helps to keep doing the same additional ( for me mechanics ) because revising for M1 was really easy after doing M2. Alot of people struggle with the first year of mechanics because it is so different, so i'd do S2.