I feel, personally, that statistics is slightly harder than mechanics, but with statistics, it's all about data collection etc., I'd say go for mechanics and with pure it's tough although once you've grasped the concept you can do it with flying colours. But is there mechanics and pure offered separately? I know Statistics is an A-Level offered by itself but I've never heard of A-Level Pure Maths or A-Level Mechanics. Usually Pure and Mechanics are offered alongside Statistics under A-Level Mathematics