The iPad is better in terms of being an RGB LCD over a Pentile OLED, as well as the 120Hz display, but the iPhone has higher pixel density and @3x rendering. Also the iPhones black levels/ contrast are better for HDR content.
Overall assuming you aren’t sensitive to the PWM flicker or the pentile characteristic, probably the iPhone would look technically better. It does depend on what you’re looking at though, fast moving content, particularly if it is >60Hz would look a lot less blurry on the iPad. Static images should have punchier colours and better contrast on the iPhone. HDR content will look better on the iPhone.
I guess the last thing to mention is size, a 12.9” iPad Pro is just going to be bigger (and in this case that does mean better) for watching a movie or something over even the Pro Max iPhones.