few things that's not been mentioned yet.
do you wifi when out and about?
with BT you can use the free hotspots, just login using your BT details.
plusnet used to use the same BT hotspots, but stopped early last year.
so if you want the free BT hotspot wifi usage, then youll have to go BT.
otherwise plusnets good, even get an option to own a domain based off your username for £1 a month. mines <my username>.plus.com and I have unlimited email addresses based off that, like stevie@<my username>.plus.com (BT may aswell, not checked)
plusnet do not throttle in anyway either. me, dad and sister have been doing youtube/steam/iplayer and reach 2TB a month on average, speeds been 1.6MB/s pretty much constant throughout from the 16meg line.