Best answer: The OnePlus 6T is effectively the same no matter where you buy it, but the T-Mobile version has a couple of restrictions. It doesn't have dual SIM functionality, is SIM locked when it's financed, and only comes in one color, RAM and storage configuration. Unless you want to use T-Mobile's financing or trade-in offers, you're best buying a 6T directly from OnePlus.
OnePlus: OnePlus 6T ($549)
The OnePlus 6T is mostly the same no matter where you buy it
Here's the good thing: the T-Mobile "version" of the OnePlus 6T is effectively identical to the unlocked model when it comes to its hardware. It has all of the same specs, features and capabilities. T-Mobile isn't changing the software on the OnePlus 6T, and only pre-installs two necessary apps (the main T-Mobile app and a SIM unlocking app), so you don't have anything to worry about there. It's fully functional.
On the other side of the equation, the unlocked OnePlus 6T works great on T-Mobile too, including VoLTE calling and Wi-Fi calling.
There are a few T-Mobile optimizations, and restrictions
The first thing you get for buying a OnePlus 6T from T-Mobile is proper optimization for its network. Every OnePlus 6T is certified to work with T-Mobile, and the unlocked models have the same network bands, but T-Mobile has spent extra time to tune and test the phones it sells. How will that affect your real-world use? It's hard to tell, really — but you can know you'll never have an issue with a OnePlus 6T purchased from T-Mobile, and the carrier will back up and support the phone it sold being used on its network.
T-Mobile's additions are minimal, but so are its restrictions.
The trade-off for that optimization is a couple of omissions and restrictions. Like all other T-Mobile phones, the OnePlus 6T is SIM locked so long as it's being financed by the carrier. You'll need to finance the phone in order to get its great trade-in savings, so that's something to keep in mind. T-Mobile makes it easy to request an unlock using an app, but you have to meet the requirements — basically the phone has to be active for 40 days, paid off, not reported stolen and attached to your T-Mobile account in good standing.
For the tinkerers and gotta-have-it-now Android fans, T-Mobile's OnePlus 6T has extra restrictions. The carrier's version cannot use the OnePlus Open Beta software builds, so you'll have to wait for the official software releases to get all of the latest features on your phone. Those releases may come a little slower as well, considering they need to go through the extra level of certification and testing for T-Mobile's network — though they are assuring customers that their partnership will get those updates out as quickly as possible.
The only other shortcoming is in your configuration options. T-Mobile only has the OnePlus 6T in the "Mirror Black" finish, not the sleeker Midnight Black. It also only comes with 8GB of RAM and 128GB of storage. Pricing is the same, $580, no matter where you get the 8GB/128GB model, but if you want to save the $30 and get the base model, or spend $50 extra for 256GB of storage, you'll have to buy from OnePlus.
Unless you need to financing or trade-in deal, you should buy unlocked
All of that leads up to this: which one should you buy? Most people should buy the OnePlus 6T unlocked. When you do, you get to choose between both colors and multiple RAM and storage options. You also get a hassle-free unlocked SIM slot, while retaining full T-Mobile compatibility.
If you plan on staying with T-Mobile, you can benefit from its financing and trade-in offerings.
But there are still good reasons to just buy from T-Mobile. The OnePlus 6T costs the same no matter where you get it, but T-Mobile offers 24-month no-interest financing to make it even more affordable. The carrier also has trade-in deals so you can save hundreds of dollars on the up-front cost when you bring in an old phone. No matter how you feel about the restrictions of this model, that may make up your mind right there.
If you plan on staying with T-Mobile for the life of the phone, you won't care about its restrictions of an initially locked SIM slot and limited version choices. If you want to sell the phone later, you'll prefer to have an unlocked model to ease that process. And of course if you want to save $30 up front with the base model, or spring the extra $50 for 256GB of storage, you'll need to buy from OnePlus.
Buy unlocked if you can, but know you know the restrictions ahead of time T-Mobile is a fine choice.
When you buy the OnePlus 6T unlocked, you get to choose different colors, RAM and storage options. You also don't have to deal with having a locked SIM slot. You just miss out on T-Mobile's financing and trade-in deals.