I'd recommend one for 3U maths, physics is probably not as important however. I'm not sure what they do at Usyd, but at UTS there was a maths diagnostic test and if your maths wasn't up to scratch you would have to take a lower level of maths in your first semester as an elective.
It's probably best to go to the bridging course for the uni you've enrolled in, the material will likely be tailored towards the subjects you will taking in first year, and it gives you an opportunity to meet people you will be studying with.
There might be a minimal difference in cost, but I don't think it will be a big deal (e.g. back in 2012, I think UNSW charged around $200 while UTS had free bridging courses for incoming students)