dark_angel said:
Things your brain lack.
512 adsl is speed-limited to 512kbits/second, or 64kbytes a second.
Cable, when it reads 'unlimited', is not speed-capped like ADSL. it has a hard limit of around 10Mbits/sec, or 1.25MBytes/second.
However, don't think that 'unlimited' means unlimited amount of downloads - on telstra cable, the moment you toe over 10GB of data, you'll get shaped down to 64kbits/sec, or 8kBytes/sec, and thus download only a tad faster than a dialup modem.
Whilst ADSL tends to be slower in speed, they almost always have higher download limits. If you're a leecher, go ADSL. If you know you won't hit 10GB of data in a month, but when downloading stuff you want it lightning-fast, go cable.
Pretty much a simple rule of thumb, though it is about to change in the next couple of months (read: look at whirlpool's iiNet and Internode news reports.)