I kept my iPhone 6+ for almost 7 years but it was time for an upgrade. Support for this model stopped at iOS 12 (the 6S is still supported by iOS 15) and I'm unable to update many apps to current version. Also my daughter dropped the phone one too many times and the touch screen would sometimes become unresponsive & required reboot.
For T-Mobile customers the old iPhone 6+ is worth $100 in trade-in, but I opted to keep mine as backup. If you choose to order from T-Mobile web site you'll need to wait until end of Oct for delivery. But if you call around to local T-Mobile stores some have the new iPhone 13's in stock. Selection is limited -- my local store only had the gold colored iPhone 13 Pro Max 128 GB ($1,100). I'd preferred to pay $100 more for the 256 GB model, but didn't want to wait 5-6 weeks for it.
If you're upgrading from an old iPhone, the improvements in screen display, app speed and camera photo/video is significant. The larger camera lens produce much brighter photos/videos in higher quality. Physically the iPhone 13 Pro Max is about the same size as the iPhone 6 Plus, except the larger display screen covers the almost the entire surface area.
If you currently own an iPhone 12, there's little reason to upgrade to the 13 -- it's basically a slightly upgraded iPhone 12. If you're visiting T-Mobile store you can also play with the new Samsung Galaxy Z Fold 3. The larger fold-out display looks great, but I would not recommend paying $1800 to be a beta tester for this product (like I did with the original iPhone in 2007). Give it a few months to read owner reviews. I expect to see cheaper fold-out phones from other companies next year.
If you want to save money, T-Mobile has its own house brand "REVVL". The REVVL V+ 5G with 6.8" display is only $200, and the OnePlus Nord N200 5G for few dollars more with better display. These are great bargains, with microSD card expansion to add a lot of storage, and unlike Apple iPhone (cough) they come with the plug-in charger.