Welcome to Android Central! Man, that's a really tough choice. My coworker has the P20 and it's awesome! I'd say you'd get an excellent phone experience out of either device. I have a Pixel 2 and love it. You are guaranteed software updates and so that is something to consider if you do go for the 2 XL.
I recommend going to your local retailer and holding both phones in your hand, and testing out their features (messaging, camera, homescreen setups) as best as you can. If you feel like you can't decide between the two, think about which one you see yourself using for a longer period of time until your contract is up, for example.
Keep us posted on your decision!