Upgrading to the new iPhone? 5 questions to get the best deal

Apple CEO Tim Cook introduces the new iPhone 6s and 6s Plus at Bill Graham Civic Auditorium on Sept. 9, 2015, in San Francisco.
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With new iPhones hitting stores this week, some consumers are considering upgrade options with different carriers. And if you're confused by the endless options for data usage and phone payment plans, you aren't alone.

WalletHub, a personal finance website, did the math, and determined that for families, Sprint is the most affordable carrier, but for individuals, T-Mobile can be a compelling buy—as long as you pick a standard plan for your 6S.

While the WalletHub numbers provide an interesting baseline, their results were hotly disputed by other carriers, who argued that if you are, say, a video-streaming aficionado, or conversely, rarely open your mobile browser, these results may not hold as much weight.

But on top of the data, there are five questions you can ask your carrier to make sure you get the right plan.

WalletHub's study looked at the cost over two years of buying an iPhone 6S with at least 3 gigabytes (GB) of monthly data. It surveyed both different carriers and payment methods, such as a subsidized phone with a two-year contract, a leasing plan, buying the phone up front with no contract, or an installment plan from Apple or the carrier. WalletHub also separated individual plans from family plans, where the study calculated the cost of upgrading one of four phones to an iPhone 6S.

1. What's the best price?

Best cell phone plan for you?

The cheapest option for individuals was a T-Mobile leasing plan, which costs $1,881 over two years, and a Sprint leasing plan for families, at $3,401 for a family of four. The priciest choices were an individual installment plan from AT&T at $2,349, or adding a new two-year AT&T contract to a family plan, at $4,507 over two years.

Carriers were quick to point out to CNBC potential deals and exceptions to WalletHub's findings. They are being forced to provide better deals for consumers, now that phonemakers such as Apple and soon, maybe Samsung, offer financing plans, regardless of which carrier the buyer chooses.

"These plans have come a long way," said Jill Gonzalez, who assisted in WalletHub's research. "It used to be a one-size-fits-all-needs world, and it's not at all anymore. Reading into these plans and what they offer and how much they've changed, it's a big deal."

2. Should I change my data plan?

Verizon CEO on the record

Perhaps the most notable way to save or splurge from WalletHub's numbers is to adjust your data usage outside of 3GB. The average smartphone owner uses 2.9GB, reports market research group NPD Group, though that figure varies widely.

AT&T, for instance, ended up more expensive by WalletHub's measure. It's important to note, though, this may not be the case if you aren't using the web on the go.

AT&T's value plans come with just 2GB of data usage, meaning that the 3GB standard in WalletHub's study would result in $15 monthly overage fees. If you are comfortable using 2GB or less per month, you can even roll data over to the next month under AT&T's plan.

Sprint and T-Mobile, meanwhile, offered more data per line than other carriers for families, at 10GB per line, compared with Verizon's 3GB at similar prices—meaning more bang for your buck, data hogs.

Read MoreSprint CEO: Why 'iPhone Forever' plan works

3. What's in the leasing agreement?

Another caveat is that while newer phone-leasing options may seem like a great deal, there is more there than meets the eye.

WalletHub's study assumes the buyer has excellent credit. If you have poor credit, leasing options might end up being pricier for you than a traditional plan, Gonzalez said.

Apple's new installment plan includes insurance AppleCare and a new phone every year—which could be a great deal if you tend to do major damage to your devices. But it's more expensive.

Finally, as with any leasing plan, the amount that you end up paying depends on how quickly you pay it off. If you pay above or below the minimum monthly payment, the fee you have left at the end could vary, carriers said.

Read MoreReviewers applaud iPhone 6S' Force Touch and Siri

4. Can I trade in my old phone?

Another potentially big way to save is by taking advantage or promotions that let you trade in your old phone.

Starting on Friday, a Sprint spokesman said, qualified customers can get iPhone 6S for $15 per month and iPhone 6S Plus for $19 per month, or $19 and $22, respectively, if they do not trade in an existing smartphone. A T-Mobile spokesman also said if a customer trades in a phone worth more than $90, the monthly fee would be lower than some competitors'.

If you're willing to wait for a new iPhone, you could get a better deal, too. Based on past releases, Gazelle, an online service to buy or sell used personal devices, expects to start selling iPhone 6S within three to six months for about $599, CEO Chris Sullivan said.

"It's not just price, it's whether you can take your phone with you," John Bergmayer, senior staff attorney of technology and communications issues group Public Knowledge, told WalletHub. "Can you unlock your phone if you break your contract? If yes, then (possibly, depending on network compatibility) not needing a new phone on a new carrier should factor in."

5. What's the quality of service I get?

AT&T: Apple carrier change not a big deal

Verizon announced last month it would stop subsidizing phones in exchange for two-year contracts. Instead Verizon customers pay a flat fee for each phone line, and are offered four consolidated plans based on data usage. The move was similar to one by T-Mobile in past years, though in the WalletHub study, T-Mobile was more cost-effective than Verizon.

"Consumers usually tell us they make their decisions on value, not price," a Verizon spokesman told CNBC. "When you take into account the faster, more reliable performance of our network, and exclusive access services and features such as NFL mobile, customers tell us that's worth it."

To be sure, WalletHub's study also fails to take into account factors such as regional coverage, talk and text limits, cheaper phone options and customer service, all of which could sway consumer sentiment. Verizon and TracFone, for instance, scored highest in the 2015 American Customer Satisfaction Index, while AT&T, T-Mobile and Sprint scored at or below the industry average.

AT&T also disputed WalletHub's conclusions. "Due to the lack of methodology and research, WalletHub's recent study does not reflect correct figures for a variety of AT&T's plans," said a spokesperson. "For accurate information regarding purchasing an iPhone from AT&T its best to contact AT&T directly."

Read MoreAT&T to combine television, wireless services in one bill

Even beyond the four major carriers, there may be an option that works for your needs that doesn't include the iPhone 6S. Indeed, start-up WhistleOut's estimates there are almost 270,000 phone-plan combinations.

Similar studies have used other criteria to assess the value of phone plans, though they might not be available for the iPhone 6S. A June study by Money magazine found carriers like Boost Mobile, Virgin Mobile and Cricket could also be cost-effective for lightweight users.

"There are a couple of things that Sprint has going for it," Gonzalez said. "The unlimited plans end up working a lot better, especially with current promotions," Gonzalez said. "AT&T does offer the rollover data, so if you're varying for month-to-month, that would help you out. [Verizon] are focusing on deals for people who have been there for years and remaining with them, where Sprint has done the exact opposite."