Internet (pre NBN) can be a tricky beast. While it’s not going to be applicable to you, get Cable if you can! Why someone living in a cabled area would get ADSL instead is beyond me, although I will concede that a congested neighbourhood and a close exchange could make ADSL the better option. But I digress.
As has been said, with ADSL your speed is down to line length and line quality. All things being equal it doesn’t matter which ISP you’re with the line will make the difference about 90% of the time to the speed you will get. The further you are away from the exchange the higher that percentage is going to get since the line will be what slows you down.
If your local exchange is over subscribed (ie too many people crammed onto one provider) it can go to hell very quickly. Usually Telstra have better investment in the exchanges so have better capacity AND being the “premium” priced offering tend to have less congestion (vs the cheap companies with little investment and $10 unlimited plans who all the heavy downloaders flock to).
BUT, and about 99% of the reason I moved back to Telstra until recently was that they own(ed) the copper lines. If you have a problem with the lines and you’re with anyone else they would have to log a call with Telstra to come out and fix the lines. Worse when I was with Exetel (who were an Optus reseller) and they would ring Optus who would in turn ring Telstra who would send out a technician. By cutting out the middle man and moving direct to Telstra I felt like I got far more attention and care in actually fixing things properly.
Of course if your lines are good and you don’t have issues it shouldn’t matter who you choose to go with.
NBN sort of levels the playing field with NBN Co owning all the infrastructure and everyone (including Telstra) reselling the same thing. This makes the choice more about price and inclusions rather than worrying about if it will work properly (contention issues aside).
tll;dr Telstra are probably the best option albeit the most expensive.