Good comparison. They are alike but slightly different. The Range Rover is the equivalent of an Egyptian tomb on wheels: It's massive and tuned for maximum comfort, while the Bentayga is more of a crossover like the Cayenne.
Rolls-Royce and even Lamborghini must be rubbing their hands at the opportunity to produce something truly exceptional. While the Range Rover is great, it's very common and will for some not feel special enough. The Bentagya has the specs but no matter how much you like it, you've got to admit that it's not the most harmonious looking car.