OnePlus 6T vs Galaxy S9+: Cameras
While we're comparing the specs, one important area we need to talk about is the camera. Or the camera(s).
The camera is always one of the weakest links for OnePlus, but that's no longer the case this year. They're using a bigger 1.22um sensor with a fast f/1.7 aperture, and it finally has optical and electronic image stabilization. At least the main camera. The secondary lens is 20MP and helps take portrait-mode photography and a few other things. However, that's what we got with the OnePlus 6, and the company didn't make any improvements for the 6T.
Basically, the S9+ still has a better camera. It's not amazingly better, but it's better. Even the single S9 camera will produce better shots.
Samsung's Galaxy S9+ has an excellent dual camera setup with 12MP cameras. Both are 12MP, both have optical image stabilization, and one has a variable aperture camera to get the best lighting in any situation. Samsung's also using a larger sensor size that captures more light, quicker, and has a better autofocus system. The Galaxy S9 series is one of the best for those that like photography.
That said, the OnePlus 6T has the best camera we've seen from OnePlus. It's won't be Samsung or Apple good, but it's close, and more than good enough for most people. You'll just need to decide how much extra you want to spend for those improvements.