Cheaper than BT and back end infrastructure a little different despite the same cables to the exchange. Premium products possibly better options for no throttling.
Prices are based around the openreach pricing based on exchange so if you have a popular exchange and are close by you get a discount. Some providers absorb the difference.
Best is support. BT are okay but can have their frustrating moments and a bit difficult with billing issues. PlusNet go out of their way to help, and their support forum has very helpful staff. That said, issues with your line are referred to BT engineers so no difference there for good or bad.
Their routers might not be amazing for wifi signal though maybe that’s changed, but you can use other routers or a wireless access point. Their routers don’t try to share your connection with the world though like BT hubs do.