iPhone 6S and 6S+

Because it's an "S" year upgrade and because the iPhone was just redesigned, the next-generation version will focus on internal improvements rather than an updated external look. Screen sizes will remain at 4.7 and 5.5 inches, and Apple is not expected to introduce a new 4-inch model.

There may be a few exterior changes, though. There's been a rumor that Apple will add a new color option to its iPhone lineup in 2015 -- pink (which may be rose gold). We've also seen two rumorssuggesting Apple might opt to use the same 7000 series aluminum used in the Apple Watch in the next-generation iPhone. The aluminum is 60% stronger than standard aluminum but still lightweight.

In the past, "S" upgrades have brought features like Siri, Touch ID, new processors, and camera improvements, and we can expect to see many of the same updates with the iPhone 6s and iPhone 6s Plus. According to rumors, the new devices will gain a faster A9 processor, 2GB of RAM, a 12-megapixel camera with 4K video recording, an additional microphone to enhance voice quality, and potentially a new internal mechanical design to address some bending issues users experienced with the iPhone 6 and iPhone 6 Plus.

The 2015 iPhones are also expected to gain the Force Touch feature first introduced with the Apple Watch, allowing for new gestures that incorporate pressure sensitivity, and improved Touch ID to make fingerprint recognition faster. A report from analyst Ming-Chi Kuo has also suggested Apple may use sapphire in a limited number of iPhone 6s Plus units, "if drop test issues can be resolved."

Though the iPhone 6s and 6s Plus are expected to retain the same design as iPhone 6 and 6 Plus, the addition of Force Touch and 7000 series aluminum could slightly change the thickness and the dimensions of the iPhone 6s and the iPhone 6s Plus.

Multiple rumors and leaked schematics, and even what's said to be a fully assembled "prototype" iPhone 6s, have suggested the iPhones could be slightly thicker, by approximately 0.2mm. Renderings of the devices and a video comparison of the iPhone 6 and 6s shell indicate the iPhone 6s may be 7.1mm thick instead of 6.9mm while the iPhone 6s Plus may be 7.3mm thick instead of 7.1mm. At 0.2mm, the difference between the iPhone 6 and 6s will be nearly undetectable and most cases and accessories designed for the iPhone 6 should continue to work with the iPhone 6s.

A materials analysis of the iPhone 6s shell has confirmed that Apple is indeed using a new alloy for the device. With 5 percent zinc, it's in line with many 7000 Series aluminum alloys. A bend test on the shell suggests it's much stronger and more resistant to bending, and in addition to using a new alloy, Apple has also added reinforcement to the areas around the home button and volume buttons of the device.

The iPhone 6s and 6s Plus will continue to offer features that have become integral to the iPhone, including NFC for Apple Pay, 802.11ac Wi-Fi capabilities, and LTE Advanced. The two new phones will likely be available in the same 16, 64, and 128GB storage options as the iPhone 6 and 6 Plus.

It's an "S" upgrade year, Apple has asked its suppliers to produce a record-breaking 85 to 90 million units of the iPhone 6s and iPhone 6s Plus combined by the end of the year. The high number of orders suggests Apple is expecting significant demand for the two devices and hoping to avoid supply shortages, but the addition of Force Touch may impact initial production numbers.

Apple plans to unveil the iPhone at a media event in San Francisco scheduled for September 9. Pre-orders, if accepted, will follow on September 11, with an official launch likely happening on September 18.