Not sure about Sprint, but with T-Mobile having native coverage in the Black Hills (Belle Fourche, Spearfish, Sturgis, Rapid City, Deadwood, etc.) by year end I suspect TMobile may have a COW for Rally since they have the spectrum and such here now to do all of that. But it's hard to say at this time for sure. As for Sprint...who knows. I've seem a lot of crazy speculation online. I suspect they however will still have poor coverage, if any, during the Sturgis Rally this year. I would plan on using at least a prepaid that uses Verizon or ATTs towers. TMobile phones will work fine and just use ATTs towers. Voice and texting will be fine. But be aware of data roaming if they are not yet here by then. So you may have very limited data usage unless on WiFi.
I live in the Black Hills.