Illumina (NASDAQ:ILMN) has been -- and still is -- a leader in one of the most important scientific advances in human history. The company's technology helped usher in widespread use of gene sequencing, which has led to greater understanding of many genetic diseases. As a result, Illumina has been a fantastic growth stock, with its share price soaring nearly 900% since the company's IPO in 2000. 

But another company has stood on the shoulders of Illumina. Exact Sciences (NASDAQ:EXAS) markets the Cologuard DNA test for colorectal cancer. Without the progress made possible by Illumina, Exact Sciences might not even exist. However, despite Illumina's impressive past and great prospects for the future, Exact Sciences is the better growth stock. Here are three reasons why.

Businessman pointing to chart lines going upward

Image source: Getty Images.

1.Stronger current stock and sales growth

This might fall into the "no duh" category, but one of the most compelling reasons Exact Sciences is a better growth stock than Illumina is that it's...growing faster. That's certainly true for the stock price.

ILMN Chart

ILMN data by YCharts.

It's also true for sales growth -- at least on a percentage basis.

ILMN Revenue (Quarterly) Chart

ILMN Revenue (Quarterly) data by YCharts.

Illumina leads in earnings growth. However, that's because Exact Sciences hasn't turned a profit yet. The company is headed in the right direction, though. In the first half of 2016, Exact Sciences lost $92.2 million. The company's net loss in the first half of this year was a much-improved $65.7 million. 

2. Faster long-term growth than Illumina 

One objection to the first point is that it reflects only a recent snapshot. That's a fair observation, so let's look at growth for the two companies over a longer period.

ILMN Chart

ILMN data by YCharts.

As the above chart shows, Exact Sciences has been growing faster than Illumina over the last 10 years. Granted, though, it was neck-and-neck through much of that period. But what about sales growth? Looking at a 10-year period would be skewed, because Exact Sciences didn't win FDA approval for Cologuard until August 2014. However, let's compare sales growth for the two companies in the first three years after their initial product launches.

ILMN Revenue (Quarterly) Chart

ILMN Revenue (Quarterly) data by YCharts.

EXAS Revenue (Quarterly) Chart

EXAS Revenue (Quarterly) data by YCharts.

Exact Sciences also beats Illumina in sales growth at similar stages in their product launch cycles. There were different economic factors at play in the early 2000s than there are now, of course. However, it's hard to argue against the case that Exact Sciences is the better growth stock than Illumina was at similar stages.

3.Different dynamics

Exact Sciences is also a better growth stock than Illumina because it faces different dynamics. Typically, companies experience stronger growth earlier in their product launch cycles. It's harder to achieve rapid growth for a large, stable company than it is for a smaller company that is only beginning to tap its potential market.

Exact Sciences, for example, estimates that it can capture 30% of the colorectal screening market. If the company can achieve this goal, it would represent annual revenue roughly 20 times greater than Exact Sciences currently generates. Exact Sciences is also developing liquid biopsies for early detection of multiple types of cancer. This presents yet another significant opportunity for growth. 

Illumina, on the other hand, faces a more mature market. Many organizations that would benefit from gene sequencing have already purchased systems from Illumina. The fact that roughly 65% of Illumina's total revenue stems from recurring sales of consumables underscores how much progress the company has made in winning customers.

The growth story isn't over for Illumina by any stretch, though. Illumina's recent launch of its new NovaSeq system should boost sales considerably as customers with older systems convert to the new system. NovaSeq could also expand the market by lowering costs for gene sequencing.

And while Exact Sciences appears to have better growth prospects, the company also has more risks than Illumina does. Other rivals could beat Exact Sciences in developing effective liquid biopsies. It's even possible that these biopsies could hurt Cologuard's prospects. Illumina, meanwhile, enjoys a solid advantage over rivals that is less likely to be significantly diminished. Higher growth prospects often go hand-in-hand with higher risk levels.

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.