All three phones look nearly identical.
The OnePlus 5 and OnePlus 5T have nearly identical metal backs. The 8GB RAM model of the OnePlus 6 has a glass back with a matte texture that looks nearly identical to the metal finish from previous OnePlus devices.
The 6GB RAM model of the OnePlus 6, meanwhile, has a glossy glass back that clearly looks different from the metal backs on previous OnePlus smartphones.
On the front, the only major difference is the bezel sizes.
It's true that the OnePlus 5 has significantly larger bezels than the OnePlus 5T and OnePlus 6. But you'd have to ask yourself if your ready to spend a minimum of $530 for slightly better performance and narrower bezels.
The OnePlus 5 and OnePlus 5T are still top performers.
The OnePlus 5 and 5T run on Qualcomm's Snapdragon 835 processor, which was the top mobile chip turning the wheels inside the top smartphones of 2017. Meanwhile, the OnePlus 6 runs on the newer Snapdragon 845 chip from Qualcomm.
On paper, the Snapdragon 845 is a faster chip than the 835, but I don't feel a significant difference in speed when using the OnePlus 5/5T compared to the OnePlus 6.
All three phones have pretty much identical features.
Feature-wise, there's very little that distinguished the latest OnePlus 6 from the OnePlus 5/5T. All three phones come with OnePlus' Dash Charge that charges your phone at full speed, even when you're using it while it's charging.
All three phones also have dual-camera systems, and all are capable of taking portrait shots with the rear camera. In fact, you could say the older OnePlus 5 from June 2017 has a feature that the newer OnePlus 5T and OnePlus 6 don't: 1.6x optical zoom. The secondary lenses on the newer OnePlus 5T and OnePlus 6 are only used for the portrait mode effect.
Software-wise, you'll find pretty much everything from the OnePlus 6 on the OnePlus 5 and OnePlus 5T.
The OnePlus 6's camera isn't that much better than the OnePlus 5 and OnePlus 5T's – but it is slightly better.
OnePlus has toned down the brightness of the OnePlus 6 camera to prevent overblowing brighter parts of a photo. You can see more color and detail on the brighter building in the OnePlus 6 shot than you do on the OnePlus 5T shot, for example.
The only noticeable difference is the extra screen space around the "notch" on the OnePlus 6.
The extra screen space you get at the very top of the OnePlus 6 – around its "notch" – are actually quite nice to have. You can either keep it on so that apps can bleed into that extra screen space, or turn it off. Even if you turn it off, the Android notification bar remains in that extra screen space on either side of the notch, which leaves you more room for your apps, videos, and photos.
With that said, whether or not you're willing to go through the effort of selling/ditching your older OnePlus 5 or OnePlus 5T and buying the new OnePlus 6 for the sake of the extra screen space is totally up to you.
