i'm in year 11 as well and feel practically no difference between the two... it's like 2u = maths, 3u = more maths. well, maybe except for permutations and combinations which took a bit of time for me to get my head around, but otherwise no, not really much difference. just beware though, taking 3u essentially means doing homework every night and having maths everyday at school, so only take it if u can dedicate yourself.