It's personal preference. They are both amazing flagship phones.
The iPhone 7 has the perks of IOS which is very simple and minimal, IMessage, FaceTime, The entire App Store, Siri, the Airpods (which are actually decent headphones tbh), and the best part. It comes in black, black, and very black.
The Pixel is the best Android phone on the market. Sure some can say the Nexus 6p still holds up very well. Which it really does. But the Pixel is still personally my favorite Smartphone. You can get thing like. The Google play store, Google Now, The Google Assistant, Many new modifications like a different launcher, Custom OS's in general, a headphone jack, and it come in white, black, and JESUS CHRIST blue.
It honestly depends on what you are looking in for a phone. Both run extremely smooth, and well. Some other great selections for phones are the Galaxy S7 Edge, One plus 3T, Nextbit Robin (I like the design), and the Xaimi Mi Mix Oh and the battery life for the regular pixel. Holy shit is it amazing.