I moved the other way. I have found performance pretty similar between the two, except that I was on BT Infinity 2 (80/20) and am on Plusnet 40/20. Latency is much better for me on Plusnet, as BT routing went Winchester->Sheffield->London before leaving the BT network, adding around 9ms to pings.
Plusnet service seems better; but BT is usually OK as well.
Not being a sports fan, free BT Sport was of no interest to me. FON was the only special BT service I liked; I could get that from Plusnet too but I'd need to buy the FON router.
It is a shame that loyalty offers never match up to the disloyalty ones.
Recently moved from BT Infinity 2 to PlusNet. Very happy so far.