Without going all tecchy on you, it all depends on the area and how far you are from your exchange.
Areas in the UK with 20mb broadband use what is called fibre to the curb technology i.e. they are fibre optic to the little BT box at the end of your street/in your area and then copper wire from there to the home. Where broadband is slower, it is all copper wire. Copper wire reduces its effectiveness with higher users and therefore genuinely gets slower the further you are away from the box as people can highjack your line.
Therefore, if you are close to the box, it is worth going with high speed providers as you will genuinely notice a difference. If you are far away from the box, towards the end of the line, it doesnt really matter who you are with as you are likely to have poor connection anyway.
Finally, BT sell "seats" on the Openzone network. These seats are prioritised. It is my understanding that Talk Talk and Plus Net buy cheaper, low priority seats. Thus if there is someone going in on a Sky or O2 seat (more expensive, higher priority) they can override your connection as you get bumped down the priority list. In that sense, you really do get what you pay for.