I've had two days of downtime in seven years with Plusnet, and they were caused by a BT engineer failing hard, if we want anecdata :-)

My personal reasoning for being happier with Plusnet (or Greenbee) is that while they're owned by BT they are easier to get in contact with, don't send you to callcentres with people who don't speak fluent English or palm you off, and it's often easier to cast Summon BT Engineer if you have a quasi-independent ISP to do it for you than having to contact BT directly. A layer of insulation from the fail, perhaps?