The Pixel 8 is powered by Google's new Tensor G3 chip, while the iPhone 15 is powered by Apple's A16 Bionic chip. Both chips are expected to be incredibly powerful, but the A16 Bionic is likely to have a slight edge in terms of performance.
The Pixel 8 and the iPhone 15 are both excellent smartphones. Ultimately, the best smartphone for you will depend on your individual needs and preferences.