Generally, when Apple introduces new iPhone models, they usually bring out a newer iOS version too. While these are tested extensively on the new model(s) being released, there is no guarantee that the software will run flawlessly on your current model. Almost everyone I have talked to that has an iPhone 7 that upgraded to iOS11 regrets it. I'm seeing reports of additional battery drainage of up to 60%.
What this means is if you buy an iPhone 8 or 8+, it will be running iOS11 and you will likely be okay. It's the folks with the iPhone 7 series, 6 series and especially the 5 series that should hold off doing this upgrade. Whenever a new operating system or upgrade comes out, the first thing I do is google it. "Problems with iOS11". This time around, I found many iPhone 6 and 7 users that had done the upgrade had a giant loss in battery capacity.
If you have an iPhone 6 or before, you really need to upgrade the device, especially if you are a T-Mobile subscriber. That's because the iPhone 6 and older models did not yet support T-Mobile's 700mHZ band. The 6s model did offer 700mHZ support. Unfortunately, none of the new models including the not yet released iphone X will support T-Mobile's newest 600mHZ frequency, aka "Band 71". In a perfect world, we would be able to buy an iPhone X that supports Band 71 in November when it comes out. After all, the lower the frequency the better it works indoors. Until last year's $435 million purchase of 700mHZ spectrum, that was TM's achilles heal. Early this year, T-Mobile dropped $8 billion to be the exclusive major carrier offering 600mHZ bandwidth. AT&T, Sprint and Verizon should be thankful that Apple isn't supporting this yet. When they do, the other carriers are in big, big trouble.
Today, the only manufacturer that has a model supporting this new technology is LG. I suspect Samsung is working on this. I'm guessing that Apple will launch an "S" model early next year adding support for Band 71. Until then, I can't see dropping four figures for a device that lacks something this critical.