"The only thing that's changed is everything."

That's the tag line for the new Apple (NASDAQ:AAPL) iPhone 6s and 6s Plus, and it's a bold promise for an off-year "s" update. Ultimately, it's a boast the company doesn't deliver on.

Don't assume that means the company hasn't delivered an impressive new phone. Apple has once again topped itself in creating a standard-bearer that other companies will seek to emulate. It's just that nothing is very different, and the new signature features launched with iPhone 6s are underwhelming or at least underutilized.

iPhone 6s and 6s Plus are a contradiction. They may be the greatest smartphones ever created, but if you have their predecessor, the iPhone 6, there is very little reason to go and get one.

Source: Apple.

A bit about Apple's product cycle
Apple releases a new iPhone every year, usually in September, but it only updates the look of the device every other year. The "s" models retain the physical characteristics of their predecessor but offer improved specs. In this case, there are improved front and back cameras, as well as a new processor running things -- a custom-designed 64-bit A9 chip that the company says delivers up to 70% faster CPU performance, and up to 90% faster GPU performance.

For the "s" releases, the hook isn't the improved specs as much as it is the launch of a killer feature. In the case of the first "s" model, the iPhone 4s, it was the launch of Siri -- a truly revolutionary-seeming feature that made the phone a must-have.

In this case the big new feature is 3D Touch, an idea that didn't seem that necessary or useful before the launch -- which turned out to be true once I put it to the test.

What's wrong with 3D Touch?
Before sharing my complaints about the new touch technology, let's consider that Apple might be overselling it a bit. Here's the promotional copy from the company's website:

The original iPhone introduced the world to Multi-Touch, forever changing the way people experience technology. With 3D Touch, you can do things that were never possible before. It senses how deeply you press the display, letting you do all kinds of essential things more quickly and simply. And it gives you real-time feedback in the form of subtle taps from the all-new Taptic Engine.

The first part of that statement is true. Multi-touch, which allows pinch-to-zoom functionality, was a big deal, and it paved the way for pretty much every smartphone that came after it. 3D Touch, however, is a whole lot less revolutionary. It does let you do things that weren't previously possible, but those things just aren't that necessary or useful.

3D Touch, for example, lets you bring up a shortcut menu for many apps by pressing on the app a little harder. It may be my clumsy nearly 42-year-old fingers, but my success rate for doing that is about 50%. On the new Apple News app, the correct touch brings up a menu that brings me to my favorite content faster. A slightly wrong touch makes all the apps on my screen shake like they're doing the twist.

You can see where an engine that reads the various degrees of pressure used in a touch might be useful in games or other areas, but as currently constituted it's a novelty at best and a nuisance at worst. It does not, at least for me, have the wow factor of Siri, and it's not a reason to upgrade.

The rest of the new features
In addition to 3D Touch, the new iPhones added a number of other new features. One of the more highly touted additions, Live Photo, essentially turns your pictures into mini-videos with sound and motion. It works and it's sort of fun, but it also falls into the category of pleasant novelty.

One new feature that launched with the 6s, "Hey Siri," is actually an iOS9 feature that can be added to older-model iPhones by upgrading the operating system. It's a small change, but the new technology lets you activate Apple's improved voice assistant simply by saying "Hey Siri."

It's a small change, but one that's very appreciated while driving. For example, on the way to the coffee shop where I'm writing this, I said "Hey, Siri, find me a coffee shop," and it brought up some choices, making it easy to get to a directions screen with very limited touching.

It's a great phone
Having used an iPhone 6s Plus since the new phones were launched, I'm pleased with the experience. That said, I've noticed almost nothing different from my previous iPhone 6. That's not a knock on Apple; it's a compliment that the company already had such a powerful device.

I'm sure there are times when the fast processor and improved camera (the rear-facing 12MP one for me, as I have never taken a selfie) have benefited me, but the changes are subtle. If you have an iPhone 4 or 5, or a competing phone, then run, don't walk to upgrade to 6s or 6s Plus. If, however, you have an iPhone 6, there is no compelling reason to upgrade, and it makes sense to wait for the iPhone 7.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.