Each machine has its place. If excavation is the primary task, the excavator will do it faster for a number of reasons. When you need to move the machine, you simply push the pedals or levers on the excavator instead of raising the stabilizers, swinging the seat around, shifting into gear, move the tractor, shift to neutral, swing the seat back around, and drop the stabilizers and resume digging in the backhoe. The excavator is also more maneuverable and as such can be positioned quicker. With 360 degree rotation, you can place your spoils pile in more convenient locations than the backhoe.
Where the backhoe excels is when you must transport materials or load a truck from a stockpile.