While the off-year Apple (NASDAQ:AAPL) iPhone refresh doesn't generate as much interest as when a completely new model gets released, it still creates quite a frenzy.

Even though the new iPhone 6s and 6s Plus offer only incremental improvement over their predecessors, demand will still be high. The question, of course, is which company has the best deal on an upgrade.

That's a confusing question made even more so by two factors: Apple's entry into the financing game, and Sprint (NYSE:S) and Verizon's (NYSE:VZ) recent decisions to drop the traditional two-year-contract subsidized phone model, following in the footsteps of T-Mobile (NASDAQ:TMUS), which made that move a while ago. AT&T (NYSE:T) still offers subsidies and two-year contracts, but it couples those with higher prices on service, so the company essentially removes itself from the best-deal conversation.

If you're looking to upgrade to a 6s model from an earlier iPhone at the best price, the conversation really only involves T-Mobile and Sprint.

Why not Apple, Verizon, or AT&T?
To put it simply, AT&T and Verizon are simply too expensive. Both companies charge more for service than Sprint and T-Mobile, and both charge $27 per month on a 24-month finance deal. Apple's finance offer starts at $27 a month and moves up to $32.45 if you want the right to upgrade to new models as well as Apple Care.

AT&T does offer the phone for $199 on a two-year contract, but on a 2GB data plan the subsidized user will pay $70 a month for service, while someone on an installment plan will pay $55 for the same plan. The company also further confuses its deal by requiring 30% down ($195) and then financing the remaining $454.99 over a peculiar 28 months (which does create an artificially lower monthly payment).

Apple's offer is very straightforward, and the manufacturer is offering an unlocked device, leaving customers free to search out the best service deal, but it's not the cheapest offer.

Sprint and T-Mobile are cheaper
If your goal is to obtain an iPhone 6s at the cheapest price, both Sprint and T-Mobile offer lease deals at well below the $27 a month over two years, which roughly adds up to full retail price for the 16 GB version. The offers, however, are not equal and can be a bit confusing.

On a pure price basis, Sprint offers the cheapest monthly price on an iPhone 6s through its iPhone Forever with trade-in program. Customers who bring a smartphone to trade will get the new iPhone for $15 a month on 22-month lease with the right to upgrade to a new model (though the pricing could change if you do that). If you don't have a phone to trade in, Sprint will charge $22 for the same program.

T-Mobile, through its Jump! On Demand offer, will lease customers the base model new iPhone for $20 a month on an 18-month lease. "After your 18 monthly payments, you can hand back your phone and pay nothing more. Or you can pay just $164 more if you want to keep your iPhone 6s," CEO John Legere wrote in a blog post. "That means your total cost to own your phone is just $524 -- that's a screaming deal. It's special introductory pricing for our launch, and it won't last long."

On the Jump! offer customers can get a new phone up to three times a year, though the deal price may be forfeited and the monthly cost could increase.

Source: T-Mobile's Twitter feed

Which deal is better?
The major difference between the two offers -- and it's an important one -- is that Sprint's cheaper price comes without any provision for ever owning your phone. You never build any equity. It's a pure lease deal with no buyout option.

T-Mobile's $20 price not only has a buyout option after 18 months, but it also offers a total cost below the $649 retail price of the phone.

This difference only matters to people who intend to keep the phone and not trade it in for the latest iPhone in a year. For anyone who wants to always have the latest iPhone at the cheapest possible price, Sprint has a better deal (assuming you have a trade-in), but it's a lease.

T-Mobile is cheaper without a trade-in and may still be a better deal than its rival's $15 offer. If you want the option of paying off the phone at the lowest possible price and actually owning it, T-Mobile wins.

In either case, Sprint and T-Mobile lead the way when it comes to upgrading to the new iPhone 6s.

Daniel Kline owns shares of Apple. He is taking the T-Mobile deal and switching from Sprint. The Motley Fool owns and recommends Apple. The Motley Fool recommends Verizon Communications. 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.