The two sacred sites have much in common. Each celebrates the Divine. Each has drawn pilgrims, seekers and the curious for millenia. Both have huge, awe-inducing structures designed to tower over and subdue the egos of those who enter that space.
But here is where they differ -- Stonehenge sits open to the elements and, imposing though its stones may be, graphically reminds us that its stones are themselves dwarfed by the true power and divinity of the Earth. By contrast, St. Peter's is enclosed against the elements, its magnificent glories all man-made. It may indeed point to its own transcendent truth, but that truth is not Nature. A building seeks to contain. An open-air temple simply seeks to acknowledge which is why, for me, Stonehenge radiates more spiritual power.