iPhone 14 Plus trade-in question

The T-life app is offering me $300 credit if I finance toward the iPhone 18 Pro Max, and if I were to pay it in full, it’s $185. According to the Apple Store app, my iPhone 14 Plus can be traded in for ~$285. Here’s the caveat, I’m likely not swapping phone plans. I am under my parents’ phone line. I’d prefer to pay off the phone in full, however, because I have the means to do so right now and I don’t want to deal with a reoccurring charge for the next 2 years. I wanted to ask because they’re upgrading as well, and when they asked about my phone, the salesman said there’s a chance that I could get a better deal (“free”) if I were to go to a T-Mobile store in person. Is this true or am I better off trading directly at the Apple Store (which lets me do same day in store pick up since they have it in stock)? Am I more likely to get a better deal showing up in person thanks to a promotion or should I take a different deal?