UNSW Handbook Program - Engineering (Civil Eng w Arch) - 3624
UNSW Handbook Program - Architectural Studies - 3261
civ + arch is more of an engineering degree (i have a friend doing this), first year is common with all other engineering disciplines, then in your second year you go more into your own stream (ie civil), and is a 4 year degree.
arch studies looks like more of a design based subject with no engineering involved (from what i can tell by the subjects you have to take, i cant see any physics or mechanics or even math, lol, though there would probably be the architectural version of math, whatever that may be). Also it's a 3 year degree with an optional 4th year for honours.
From your hsc subjects i'd say the civil + arch would be better, since maths ext 1 would be very helpful. They say u need to be doing physics too, but uni physics is VERY different to highschool physics. Chemistry would also be helpful for when you do materials science and chem in second semester of first year (of civ/arch).
Also most likely better employability with a civil+arch degree rather than just an arch studies degree, but ultimately it's up to you what you wanna do.
If there's anything else you wanna know, then i'd suggest sending both faculties an email for a better answer than any of us could probably give you.