Plusnet is, of course, simply BT rebranded. (Plusnet is wholly owned by BT). So the download speeds offered by Plusnet will probably be the same as those offered by BT (unless, possibly, they use different 'contention ratios'). However Plusnet usually gets better reviews for its customer service than its parent company does and it's also cheaper.
I haven't got Plusnet but, from all reports that I have read, it is an excellent service. Plusnet is part of BT but has the advantage of having all of its Call Centres in the UK; so conversations are more easily conducted if help is required. Furthermore, there are lots of helpful articles published on-line by Plusnet and its Helpline is thought to be, second to none.
My woman is happy with it. No obvious issues so far.
I'm on BT and get slightly better speeds, but I like the quarterly bill so I'm prepared to pay fractionally more for that.