I haven't read the thread over so I don't what has and hasn't been discussed, but here's my two cents from someone who has repeated Year 12 (and is currently sitting the HSC in 2007).
I got very sick in 2006 (the year I was supposed to graduate). Didn't go to school all of Term 3 and couldn't because of a slow recovery. I was doing reasonably well at school (selective), nothing special - probably achieve a 90 UAI at the rate I was going. And I only need 85 for Engineering.
Anyway, I decided to repeat as I was in no condition to sit my HSC in 2006. And the only options I was offered was repeat outright or sit the HSC and if I screw up, repeat anyway.
From experience, I wouldn't repeat unless you had some friends in the year below, or someone is repeating with you. It's always good to have someone who goes back with you, because the conversations you have with your 'new' friends are a bit empty as you have no history together. Also you will get depressed and shit, and need to stay motivated so that you don't get to relaxed with the knowledge you already have and think that that will drag you to the HSC. As you will find there will be a point where your new peers will overtake you and you need to actually put some work in.
The second year went by even faster than the first and it wasn't as bad as first envisioned. I gained a much better understanding of complicated concepts in Physics, Engineering Studies, Maths and English in particular. And in hindsight, my mates that are at Uni don't seem 'better off' to say the least. Uni is overrated - it's just like school anyway, except you're poor!
Currently, I'm looking at a 92-95UAI which is good since I only need 85, and I've applied for the Educational Access Scheme, so theoretically I only need 75ish. If I had to make the choice again, I would get an estimated UAI (in my case) or do pathways through TAFE or something, Year 12 again was just a bit too tedious.