The OnePlus 6T outperforms the PinePhone Pro by quite a bit. The OnePlus 6T is actually a pretty good Linux phone these days with postmarketOS. I picked one up to use as a Linux phone (as well as a OnePlus 6 now, but that one's running Android). I used the PinePhone for about a year, the PinePhone Pro for about a year, and now I've been using the OnePlus 6T as my daily driver for 6 months or so. I ended up getting the OnePlus 6 (non-T) and leaving it as an Android phone though, picking up a second SIM card because I had call audio issues on both Linux phones so I told people to just call me on my Android number. I still mainly use the Linux phone for everything else because the software ecosystem works so much better for me.

Pros of OnePlus 6T:

• Much better battery life (can last most of a day with screen off)

• Much better performance

• Much better GPU - Full OpenGL and Vulkan support on the OP6T

• Much lower power consumption (doesn't get anywhere near as hot)

• Louder speaker

• Better Bluetooth (my BT headphones lag to an unusable degree on the PinePhone and PinePhone Pro when there are other devices around, but are usable on the OnePlus 6T)

• Higher resolution, brighter, OLED screen

Cons of OnePlus 6T

• Bootloader is proprietary and cannot be replaced

• Android boot and partition scheme

• Call audio doesn't work reliably, sometimes calls will have no sound

• Modem doesn't receive calls unless it is forced to 2G mode, which limits Internet speed (texts seem fine though) likely due to missing VoLTE

• Sensors (rotation, ambient light, etc) require testing an open merge request at the moment, but it does work

• No USB host mode support (software/driver limitation)

• No external display support (apparently hardware limitation)

• No headphone jack (the OnePlus 6 does have it though)

• No SD card slot

• No removable battery

• Cameras don't work at all

• No keyboard case option

Pros of PinePhone Pro

• Better Linux support overall

• More OS choices

• More bootloader choices, which include completely open options

• SD card slot

• Headphone jack

• Removable battery

• Keyboard case option

• Cameras work to a degree

• Much more repairable

Cons of PinePhone Pro

• Eats battery like no other phone I've ever used, the keyboard case is almost a must-have just for usable battery life

• Gets quite hot while eating battery

• Lacks more modern OpenGL versions, Vulkan support

• Modem has a tendency to disconnect and reconnect occasionally, even with open source modem firmware

• Call audio can have some static or distortion, especially on your end (the microphone)

• Keyboard case makes using it as a phone cumbersome

You're in luck... The OnePlus 6t is one of the best supported, non purpose built, phone for Linux with support from both PostmarketOS and Mobian!