I think it really comes down to what you are looking for in a mobile phone. Both phones have their respective pros and cons.
Honor 8 Pro - 2K display, expandable storage, better camera (and more camera options), aggressively priced.
OnePlus 5 - Very fast device, better handling (due to smaller screen size), smooth OS, very handy ALERT SLIDER.
I've heard that the Honor 8 Pro does have some heating issues when the phone is used for a long time and while running multiple apps. Also, Huwaei takes a while to provide updates.
OnePlus5 apparently does not have the above issues. Haven't heard a lot of issues when it comes to heating and OnePlus does try to roll out updates quickly compared to Huawei. Then again, OnePlus is not very well known for its after sales service. I've heard of many problems in this department.
For the prices they come in both devices are amazing in their own ways.