3-D printing has become all the rage among investors. Imagine the possibilities, imagine bringing manufacturing home from China, there will soon be a 3-D printer on everyone's desk, or so the story goes. Sometimes the market gets big eyes and draws conclusions before we even decide what technology will win or what is most important in the future of 3-D printing.

3D Systems (NYSE:DDD), ExOne (NASDAQ:XONE), and Stratasys (NASDAQ:SSYS) are fighting for market share but start-ups like MakerBot are close behind, ready to upend the established competitors. The question investors need to be asking is: What will be the one constant in the industry that we can count on in the long term?

A game of chance
Betting on the companies that make 3-D printers is a roll of the dice right now. 3D Systems' earnings today show just what a gamble it can be. Even a 45% increase in quarterly revenue didn't hit expectations, which sent shares tumbling. The reason is simple: Over the past year, the price/earnings multiple of 3-D printer makers has skyrocketed and they're now priced for perfection.  

DDD P/E Ratio TTM Chart

DDD P/E Ratio TTM data by YCharts

Even a small miss can be terrible for earnings, as we saw today. Both 3D Systems and Stratasys are priced for perfection and small misses like this exacerbate the problem for shareholders. ExOne went public without the benefit of being profitable and isn't growing as quickly as 3D Systems. The company brings even more risk to shareholders.

I think investing in the equipment maker space is risky at best and a bubble waiting to pop at worst. Long-term winners will emerge but the industry is far from mature; these companies don't yet deserve the valuations they've been given.

The key to the future
Where investors should really be watching is software. None of the 3-D technology works without the software to make a 3-D model. The best company to look at in this space is Autodesk (NASDAQ:ADSK), the well-established maker of modeling software for a variety of industries. Autodesk makes a number of different software products for varying levels of skill. From Autodesk Inventor to AutoCAD to the new Autodesk 123D for 3D Systems, there's something for everyone. The company even makes free engineering software for iOS, making quick work of standard force and motion problems.

The big advantage for software over hardware makers is that the software can be used across multiple platforms. 3-D printing may make sense for a prototype for a product, but if you want to make 50 units, then a company like Proto Labs (NYSE:PRLB) may make more sense; it can use the exact same model used to make a 3-D printed product, making scaling a piece of cake.

CAD software is also very sticky. A designer is more likely to change the tools used to physically create a product than CAD software. Companies often define software with licenses while 3-D printing or molding tools are more open for change.

Foolish bottom line
The 3-D printing or rapid prototyping boom is certainly happening and investors should take notice. But I think 3D Systems, Stratasys, and ExOne are too expensive to jump into right now. They're all priced for perfection.

Autodesk is my top pick, simply because it has more staying power than others in the industry. If you're looking for a more manufacturing based play, then Proto Labs has the ability to rapidly prototype and has a software competitive advantage as well. But in both cases, it's all about the back end. A product can go from a designer's computer to a mold without a person touching it, creating its efficiencies and allowing for lower costs.

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.