Senior Science is actually a nice 'difference' to the rest of the sciences (granted I have no experience with Earth and Enviro).
As for what subject it can help with, depends on which elective you do.
The Lifestyle Chem module is rather basic, very easy for anyone who had done year 11 chem, infact, sometimes hard because of the terminology you have to use to get the marks.
The Medical Technologies component is quite good, espescially for those with an interest in health/med/nursing, it complements the biology syllabus.
The Info Systems module goes best with IPT I've found.
and I'm doing the pharmaceutics module, which again, complements the bio syllabus (even overlaps a lot).
So, it is a nice subject. Not as easy to do well in as one may first expect (I'm talking a high band 6 here), but a less stressful subjects, which you CAN still get decent marks in later after scalling, under the proviso that you don't vegetate completely. (I'm not great model here however, I did hardly anything until just before the trial, still ranking 1st in the trial and most assessments)..
Unlikely it is offered in selective schools, I've had to do it through distance ed.