To their defense, the iPhone has a terrible antenna compared to pretty much every Android (and Windows Phone, for that matter) device. The 6s is only a step up due to band 12 LTE support.
Yeah, it's disingenuous to market that the network is "better" somehow but it's only "better" when the network uses bands no one else has ever used before. To access that "better" network, you need to BUY a new phone that costs $500-$800 (depending on iPhone model) to take advantage of the network and coverage they're advertising on TV.
Here's how it works-
First,
T-Mobile's Band 12 Network is 10-15MHz in size; depending on the license acquired in that area and also use of an interference guard-band. To compare, AT&T and Verizon's Cellular Band (850Mhz; Band 5) is 25Mhz uplink and 25MHz downlink (50Mhz in total size).
T-Mobile's primary network is in the 1900MHz PCS (60MHz) and AWS (generally 35MHz) range. With 95Mhz of airwaves, of the "old network", and 10-15Mhz on the "new network" chances are more likely you'll be on their "old network" even when using the service. The marketing brilliance is that the phone will register bars of the strongest signal it can "see". This is something Sprint has done for years. So T-Mobile is like a used car dealer who wants to sell you a car that can get to a 250MPH, and marketed that way, but they conveniently forget to disclose that the car computer limits your speed to 75Mph. Also, PCS and AWS airwaves don't travel through walls and over long distances very easily; T-Mobile's regulatory affairs (Kathleen Ham) said this in a public statement.
There are caveats-
Another thing which makes T-Mobile's network unique is that T-Mobile's equipment is located on antenna structures and prime real estate which AT&T and Verizon originally built in the 1980s and 1990s. So it's not a network T-Mobile actually built. The problem is that many of the old towers can't be zoned for multiple carriers/companies due to structure and age. Connection speeds (where available) are limited to the speed of the cable network in the area. T-Mobile uses Hybird Coax-Fibre in their network. It's cheaper than digging up roads to install fiber optics.
Next, in areas where there aren't many people, or places like WalMart that sell T-Mobile service, T-Mobile will likely forego the investment all-together.
Basically, 9-times out of 10, you'd be better off with Cricket Wireless. T-Mobile just threw money at lease agreements with American Tower (Verizon), and Crown Castle (AT&T) and had the ad company make some flashy ads because it's easier to make flashy ads than training employees.
The thing is that T-Mobile wants its customers to know that the network will get even better after the upcoming 600MHz Auction. However (and this is very important) those un-auctioned bands are still in use in many areas, haven't been cleared or even assigned yet. Replacing a 100,000-watt antenna and transmission equipment takes more planning than a T-Mobile antenna on a roof with 40-watts of power. Still, and what that means is that there doesn't exist a phone sold today, from any company, which is compatible with those airwaves that haven't even been sold.
Overall, it's definitely very clever marketing, and yes, Virginia, T-Mobile lies, but you'd need to find an attorney who knows about RF engineering concepts to file (and win) a deceptive advertising lawsuit; or a complaint at the Federal Trade Commission on the basis of false advertising.
