Ah, the age old question that I debated myself a few years ago.
The IB is for all-rounders, since you need to pick subjects from each "group". Whereas, in the HSC, you can choose your strongest subjects.
IB also has a lot of "extra" subjects, like TOK (Theory of Knowledge) and CAS (Community Action Service.. I think). You must present a TOK essay and have completed a set number of CAS hours by the end of the IB. On top of this, there is an EE (Extended Essay) due at the end of the year which can be written in any subject but must be 4000 words. In contrast, the HSC is entirely subject-based.
The reason why more girls do better in the IB is:
a) It is not rankings based. You get a score from 1-7.
b) Depending on the subject, 7 can be anywhere from 80+ to 90+.
c) Due to difficulties like the Highers (more on that later), the IB is generally composed of more intelligent students than the HSC (though there's personal choice and blah blah blah)
In comparison to the ~40 people who get 99.95 ATAR every year, there is no limit on how many can get 45/45 in the IB (equivalent to 99.95). Hence it's easier to achieve 99.95, but only if you're an all-rounder, etc.
Moreover, you need to choose 4 Higher subjects in Yr 11, with a minimum of 3 in Yr 12 (from memory). One of my best friends does Higher Mathematics, and it's quite difficult (equal to if not more than 4U). Higher Chemistry has only 1 student in my school. Higher English has ridiculous unseen essays and analyses of poems that are difficult if you aren't good at extrapolating meaning. This is in contrast to doing the HSC, where you can do simpler subjects like 2U maths, 1U religion, 2U english, etc. instead of being forced to choose some extensions.
OH and the IB classes generally finish the syllabus with a good 6 months or so to go, which is spent purely on revision, whereas some HSC subjects don't finish until just before (looking at you, Chemistry). The exams are later, too.
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Seeing as you've said you're not at the top of your cohort, unless you can dramatically pull your ranks up through sheer hard work, don't do the HSC. The IB is much fairer in marks and you will not be judged against your peers. That said, you need to be an all-rounder.
Personally I chose the HSC because my ranks were good, I was strongest in the subjects I could choose, and I could focus my attention on fewer tasks. For me, it was appropriate.
Up to you.
(Oh, and yeah, sounds like MLC by your comparison of HSC and IB girls. Info day should be coming up... if not, just ask some students.)