^ yeah you don't need 1/2 for bio or psych, psych especially i'd have to say since i didn't do 1/2. except there is always something that put you at an advantage if you remember from 1/2, like research methods from psych and some cells things for bio. but it is not necessary to have prior knowledge because you re-do it all anyway.
and doing a 3/4 in year 11 is very good, personally i think it is harder due to the fact unless you were a studier in year 10, you wouldn't have much knowledge of how to study. but the time factor makes it easier of course.. though i certainly had never done any work before psych, so the entire of unit 3 i tried to learn how to study and for my exam (which i realised my horrible study methods towards unit 4 exam which put me at a disadvantage), compared to my subjects now that require memorisation (pe could be similar to psych) which i was able to utilise skills learnt right away.
so DEFINITELY do a 3/4!