As a resident I've used both extensively for many years, I don't own a car. So no I'm not joking.
It's easy to view black cabs as something very London but they are private concerns and very expensive ones at that.
The ONLY time I got "lost" in a cab was a black one.
They needed a kick up...
Use Uber in London for the convenience. Never had a problem and although not cheaper than my local, much cheaper than a black cab. The traditional cabs have just failed to keep up. I've got a cab in Brighton on a few occasions and was shocked by the price and I'm used to paying London prices...