Well, I struggled a little with Advanced Maths in yr 9 and 10, and chose to do Mathematics in Prelim. Sounded like a good idea, but it wasn't. I ended up ranked around 20/40 with a mid 70's mark, which wasn't too bad, but I was stressing myself out more than I needed to and knew that I could easily get a band 6 in General if I switched courses.
My suggestion:
1) Chose General Maths and be prepared to REALLY work to keep up and be competitive in the course (if you want a really high UAI). I didn't do Prelim General Maths, but apparently its more of a continuation on what you do in years 9 and 10.
2) Do Mathematics and give it your best shot. There's always the opportunity to switch courses at the end of Year 11 and depending on how well you cope with 2U, you will probably find General far easier because the Year 11 2U and Year 12 General courses seem to cover some of the same stuff (or that's just what I noticed). Don't just do Mathematics and spend the 3 terms sitting there thinking 'Well I don't really need to pay attention to what they're saying cause I'm switching to General at the end of the year' .. Doesn't work that way!
The people at my school who switched to General are topping the year. I'm ranked 1st now in General with the only band 6 mark, but if you switch at the end of Year 11 my only advice would be to continue working REALLY hard and not get into the habit of slacking off and developing the mindset that 'General's a bludge, I don't have to try and I get good marks' .. Sure you get a rule sheet and you might get good marks, but its highly likely you'll bomb out in the end because everyone else has got their act together and you'll be stuck in your own mindset and false beliefs, and be less competitive.
Wow that was longer than I thought! Haha, hope it helps