To study B Psyc, you can't really do genetics as an interest because it is very strict. If you are interested in genetics, you should just do B Advanced Science or B Sci and you can do a bit of both.
If you want to do med at postgrad level specialised in psychiatry, genetics would help you more i think. really psychology is more of a science from a social science point of view. But it is up to you.
I don't think universities choices will have much bearing on the quality of the degrees. You basically learn almost the same stuffs. It is you, as a student, to grab hold of all the resources available to you. The degree you choose and the mark you gain from doing it are more important than anything. And at the bachelor level, you really can't differentiate which, out of UNSW and USyd, is better. The choice is yours.