Long removed from its days of exclusivity, Apple's (NASDAQ:AAPL) latest flagship handset, the iPhone 6, is available on all four major U.S. carriers: AT&T (NYSE:T), Verizon (NYSE:VZ), Sprint (NYSE:S) and T-Mobile (NASDAQ:TMUS).

As with any other handset, coverage and pricing vary, and picking the best carrier could depend more on location and budget than anything else. Verizon and AT&T, with the largest networks, may offer the best all-around service for the greatest number of people (indeed, they have the most subscribers).

But when it comes to the iPhone 6 in particular, Sprint and T-Mobile are offering unique perks that their larger rivals can't match.

The iPhone 6 runs on Sprint Spark
Prior to the iPhone 6's unveiling, I noted that Sprint's future could depend on one of Apple's design decisions. Sprint's superfast LTE network, Sprint Spark, was incompatible with Apple's existing iPhones, including the iPhone 5S. Had the same been true for the iPhone 6, Sprint subscribers looking to purchase Apple's newest handset would have been unable to take advantage of one of Sprint's best features.

Thankfully for Sprint, that didn't happen -- the iPhone 6 is fully compatible with its Spark network.

Unfortunately, Sprint Spark isn't available in many markets -- just 29 cities in total -- which may explain why Sprint continues to lose subscribers (220,000 customers last quarter) in spite of Spark's existence. But in the few markets where it is available, the performance has been stellar, with download speeds often eclipsing its larger rivals' standard LTE networks. In theory, Sprint Spark is capable of delivering speeds on par with the fastest landline providers, though it could take years for Sprint subscribers to see those speeds. Still, those living in an area where Sprint Spark is available could find the network the best choice for Apple's iPhone 6.

T-Mobile offers wi-fi calling
Sprint Spark isn't the only wireless feature new to Apple's latest handset -- the iPhone 6 is also the first iPhone to support Wi-Fi calling. That means Apple's iPhone 6 can make and receive calls and text messages over Wi-Fi connections in addition to traditional wireless networks. In theory, this allows for much greater coverage (particularly in difficult indoor areas like basements) as each and every Wi-Fi router could serve as a miniature cell tower.

But buyers of Apple's iPhone 6 looking to take advantage of this feature will only be able to do so through T-Mobile. The nation's fastest-growing wireless carrier is currently the only U.S. provider that supports Wi-Fi calling on the iPhone 6, at least for the time being.

Both AT&T and Verizon have said they will support Wi-Fi calling at some point next year, though neither has offered a definitive timetable. Sprint supports Wi-Fi calling on many Android-powered handsets, but not the iPhone 6 (at least for the time being).

Wireless customers with consistently strong signal may have little use for such feature, but those whose coverage is spotty could definitely benefit. T-Mobile's management claims nearly 60% of U.S. wireless customers often drop calls in their own home -- Wi-Fi calling is an easy fix for coverage gaps.

A more competitive U.S. wireless market?
Although Apple's share of the global smartphone market is relatively modest -- around 12% -- it's far more dominant in the U.S., with more than 40% of the market -- and growing. With such a large share in the U.S., the iPhone may be the single most important handset.

Apple's latest iPhone could make for a more competitive U.S. wireless market if buyers find two of its new features enticing. Sprint and T-Mobile are a long way from toppling AT&T and Verizon, but the iPhone 6's support for Sprint Spark and Wi-Fi calling should give the nation's third and fourth largest carriers a boost.

Sam Mattera has no position in any stocks mentioned. The Motley Fool recommends Apple. The Motley Fool owns shares of Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.