It's worth remembering that Plusnet is actually a trading name of BT anyway, so there's unlikely to be any significant difference between their services.
However for people who are stuck with old-fashioned copper wires carrying their broadband services (rather than fibre-optic cables) a priority should be to seek a provider offering 'ADSL2+', rather than the standard 'ADSL Max' that BT Wholesale normally sells to other ISPs (and BT provides direct to their own customers). ADSL2+ effectively gives customers a pair of phone lines bundled together, thus roughly doubling download speeds. (My own went from 5.5Mbps to 10.5Mbps when I got ADSL2+).
I doubt whether BT can actually provide their own customers with ADSL2+ because their Home Hubs are configured to provide one internet connection to the paying customer and one (via BT Wi-Fi) to any other subscriber who might be in the area. So it would seem that the 'second line' to that property isn't available for use in upgrading the customer's service to ADSL2+.