Another thing to consider if you are a T-Mobile customer is the 600Mhz long range band 71 which will probably be available in the upcoming iPhone. That band can have a huge impact on signal quality especially in buildings and rural areas. TMO is deploying it fairly quickly. TMO has roaming agreement with US Cellular so it may still make a difference in some areas.
If you are sure you will stay with US cellular I suppose it doesn’t matter if you buy an iPhone 8 or 7 but I’d still wait till the new gen for prices to fall on the older ones.