Similar to many high-end phones this year, both OnePlus phones have a Snapdragon 865 chipset and their benchmark results were comparable to other competing devices, like the Galaxy S20 Ultra and LG V60.
T-Mobile and Verizon's OnePlus 8 phones will be rated IP68 for water resistance. But the unlocked version of the OnePlus 8 is not rated. That doesn't mean it's completely vulnerable, though. OnePlus says the OnePlus 8 is as protected against water as past models. Last year, I submerged the OnePlus 7 Pro, which isn't IP certified either, in a fish tank for 30 minutes and it survived without issues.
On specs alone, the OnePlus 8 Pro is objectively the better phone and has better features. But the cheaper OnePlus 8 still maintains enough other specs like 5G, a 90Hz display, an enduring battery life and a top-of-the-line processor to deliver a premium Android experience.