I've demoed the OnePlus 6T at T-Mobile and the Pixel 3 XL at Verizon. I would definitely get the OnePlus 6T because OxygenOS has better gestures than the mess Google made with Pie, the extra RAM, smaller notch, and fingerprint on display. The issue is the screen. It completely ruins the experience.
While 1080p LCD displays look great - sharp, crisp, no visible pixels or subpixels, 1080p AMOLEDs are a different story due to the Pentile subpixel arrangement. There are visible black gaps between the subpixels which are perceived as a checkerboard pattern of white pixels on a white screen, and is most noticeable when watching light colored videos. It also shows in the form of jagged text. On black themes the problem is more prevalent, there's visible green bleeding out of the top of text and red on the bottom. Look even closer and the color bleed is jagged, the gaps between bleeding subpixels are visible. For me it's very hard to ignore these because I even notice them day to day on a Galaxy S8+ with a 1440p AMOLED.
The Pixel 3 XL has a 1440p AMOLED so it's much better. Definitely still visible but bearable unlike the OnePlus 6T's 1080p.
If you have a comparison it's easy to see, display an identical UI on a 1080p LCD phone and a 1440p AMOLED phone. Despite being more than twice the pixels, the AMOLED looks less smooth. I'm unable to see any subpixels, color bleed, pixels, gaps, or anything of the like on the 1080p LCD.
The screen is the way to interact with the device, so a bad screen makes a bad phone. At this point the dual front facing speakers, usable vibration motor, better kernel optimization (leading to a smoother phone, I tested the jitter on a friend's personal 3 XL), and camera processing on the Pixel are just a cherry on top. In my opinion this significantly outweighs the FOD of the OnePlus as well.
Development wise, the OnePlus 6T will have more custom kernels and ROMs because the price barrier is lower, leading to more developers buying the device. For comparison, last year's Pixel 2 has less Pie ROMs than the OnePlus 6 released later. However, the few released for Pixels do usually tend to be better and more stable due to the same price barrier.
Kernel development wise, Google definitely wins. OnePlus has hacky vendor patches all over the place. They don't merge CAF very often if at all. They squash all their changes so it's a mess to deal with and merge newer CAF updates. It also doesn't have nearly the optimization Google put the effort in to do. This is what makes Pixel devices so fast. They have EAS (scheduler) updates that improve efficiency and speed. Pixels boot extremely fast as well, making kernel development much less tedious. Last year's Pixel 2 finished booting shortly after the OnePlus 6 started its boot animation. And if you're nitpicking, the codenames are better than OnePlus - blueline for the Pixel 3, crosshatch for the Pixel 3 XL, and bluecross for the unified kernel. Certainly beats OnePlus' enchilada for OnePlus 6, fajita for OnePlus 6T, and no unified name.
That being said, I'd recommend getting the 6T if you don't notice these display issues and don't care about pure Android developer previews ASAP.
Don't take me wrong, this is just my honest opinion on the status of these two devices. They are both solid phones. No doubt, the OnePlus 6T wins in value. If the OnePlus 7 has an upgraded 1440p display it's very likely I would choose that over a Pixel.