T-Mobile USA, the smallest of the U.S. Big Four wireless carriers, only recently received access to the iPhone so, given that, a vacation blackout would point directly to a new iPhone. Notably, this year's retail launch will be the first one that includes a simultaneous release on all of the Big Four. The iPhone launched exclusively on AT&T in 2007, hit Verizon and Sprint in 2011, and reached T-Mobile earlier this year.
The iPhone 5S will be this year's flagship iPhone. Typically, everything we know about it is still just a rumor, but it will be a minimal change to the iPhone 5, with external changes mostly held in check as Apple tweaks the internals with a faster processor, improved camera features, and a fingerprint reader for authentication.
It's also expected to sport a gold color option, and possibly a fourth, black graphite color that some believe may simply be the black option, period.
The iPhone 5C is reportedly the so-called "lower cost" iPhone. It ship in a plastic case that will sport several colors, similar to Lumias, and will sell for somewhere north of $300 unsubsidized. Internals are still unknown, but it could be an iPhone 5 internally, to keep cost down, meaning that the practice of selling the prior year's model at a discount may end.