Sprint Corp., striving to win back users, is offering the best deal in the industry ahead of the biggest phone debuts of the year, the iPhone 6 and 6 Plus, arriving later this week. In a comparison of total costs for an iPhone 6 with a base level of 16 gigabytes of storage, Sprint's "iPhone for Life" plan is by far the cheapest of the four top U.S. wireless carriers, according to research by Jonathan Atkin, an analyst with RBC Capital Markets.