This totally depends on how you will be using the phone. But if I were to choose, I'd go for Samsung Galaxy S8+. It's because Samsung Galaxy S8+ has water-resistance, microSD card, better display, more battery, and most of all better customer service. Customer service is what I'm always looking for when buying anything, I just want to be sure I'm safe if anything goes wrong with my phone. But the problem with the comparison is on the budget. OnePlus 5T is way cheaper than Samsung Galaxy S8+.
Here are some of the specs of both phone.
They may have their differences but in my opinion, you can't go wrong with either, both are great smartphones. Which one's better will come down to your personal preference.