SDD is a pretty fun course if you're at all interested in computing. It lays down some basic fundamental ideas that will help you in University if IT is your main career path. IPT will do this too, but to a lesser degree. So, basically, if you like computers -- take SDD.
Now if you meant computer science as a whole, well...
This is a question that 3rd Year University Computer Science students are not quite sure how to answer, infact there's a whole course dedicated to what it "means" to be a computer scientist for third year students.
Sorry for the round-about answer, just know that Computer Science is not just about programming. It's a huge field with tons of variety in what you can do.
Sure it will help, but the extent it will help I can't say, having never done IT VET myself, however looking at the course requirements it seems you could happily cover the same (basic) material by taking both SDD/IPT, and IT is more for people who would only end up taking one computing subject for their HSC.
I was refering to which is most beneficial out of the computing courses and was going to talk about maths when the OP responded with what maths level he was doing