I used to have a BT business (allegedly) 20:1 contention service and a Plusnet ordinary retail service which both terminated in the same double socket and presumably ran in parallel to the BT exchange. The Plusnet service was consistently faster than the BT service. My conclusion; BT might not be the best choice of ISP.
BT Business broadband is expensive. It also has an ever growing range of additional business orientated services with it. Hence it is not really suitable for the everyday residential user.
It is not completely like that your two lines on the one line jack will not run in parallel.
The access network has been maintained and modified so much that the routing of pairs, even to the same premises can vary a lot. I have seen cases of a small building in a town where the lines came in from two separate DPs. Depending on the area this could mean seperate street routing to cabs and all over the place.
Line speeds aside, in an LLU exchange one would think speed differences are down the ISPs network performance.
In most (BT only) exchanges it all goes down the BT network before it reaches the ISPs network. In these cases, especially with ADSL Max, it doesn't used fixed contention ratios as such. I think Kitz mentioned something about bandwidth sharing being the thing now instead. I certainly always get very good speeds (average of around 1800-1900kbps/2000k profile) so am lucky not to be sharing to much bandwidth.
Also whilst indeed choosing BT Retail as your ISP will not ensure good service, one would hope it reduces the possible number of parties involved and so keeps things simpler and gets problems sorted faster.
BT wholesale and Openreach have regulatory obligations including to treat all communications providers equally, be that BT Retail, Zen, Plusnet. So on that basis regardless of whether you have a problem with BT Broadband or Zen, BTw should waste an equal amount of time on sorting it out.