Choosing between biotech giants Biogen (BIIB 0.38%) and Amgen (AMGN 1.40%) is not an easy task. Both companies have attractive pipelines and solid suites of products on the market. Though Amgen's revenue declined in 2019 while Biogen's rose, Amgen offered a better revenue outlook for 2020.

Both stocks make a great addition to a biotech portfolio, but before deciding which one is the best buying opportunity right now amid all the market volatility, let's take a closer look at their sales, products, and prospects.

A scientist uses a pipette to add liquid to a sample.

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The case for Biogen

Biogen reported a 7% increase in 2019 revenue and a 4% increase in fourth-quarter revenue. Its blockbuster multiple sclerosis drug, Tecfidera, posted 5% and 4% gains for the quarter and year, respectively. In more good news, Tecfidera, which represents 39% of Biogen's annual product revenue, recently came out victorious in a patent challenge from Mylan (MYL). That means Biogen won't face generic competition until 2028 unless separate cases involving Tecfidera end with a different verdict.

Still, Biogen's interferon-based MS drugs have seen revenue slip -- 14% for the quarter and 11% for the year. The company's next challenge is to ramp up sales of new MS drug Vumerity, which could potentially compensate for eventual declines in Tecfidera. After last year's revenue gain, investors may be disappointed in Biogen's outlook. The company forecasts 2020 revenue of $14 billion to $14.3 billion, lower than last year's $14.4 billion.

For a glimpse further into the future, I always look at the pipeline. Biogen boasts more than 25 investigational drug candidates, with five in phase 3 trials. All eyes are on Biogen's Alzheimer's candidate, aducanumab, which the company plans on submitting to the U.S. Food and Drug Administration for review early this year. Biogen surprised investors in the fall when it reinstated the aducanumab program after canceling it months earlier due to disappointing data. Biogen said a look at expanded data, indicating the drug reduced patients' clinical decline when given at higher doses, was the reason for the reversal.

Aducanumab represents risk and potential for Biogen. If the FDA approves the drug, it's very likely Biogen shares will climb. If the FDA rejects it, the stock may be in for some short-term losses. According to the Alzheimer's Association, 5.8 million Americans are living with the disease, and currently, there aren't treatments on the market that reduce clinical decline. So, the drug represents a significant opportunity.

The case for Amgen

A first glance at Amgen's latest earnings report may be disheartening. The company reported a 1% decrease in revenue for the fourth quarter and a 2% decrease for the year. Product sales also slipped as competition ate away at markets previously dominated by Amgen's older drugs. For example, bone marrow stimulant Neulasta, which used to generate more than $1 billion in a quarter, saw quarterly revenue fall 43% to $665 million in this most recent earnings report.

Now here are the positive points: Newer products like cholesterol drug Repatha and postmenopausal drug Prolia reported double-digit sales increases for the quarter and the year. And Amgen's forecast for 2020 revenue -- in the range of $25 billion to $25.6 billion -- indicate an annual gain of at least 6.8%.

What can boost Amgen's sales this year and beyond? The newer wave of products either still in the pipeline or already on the market. While four of Amgen's drugs saw double-digit declines in sales last year, five posted a double-digit increase in sales. The company expects an FDA decision in April on the expansion of prescribing information for psoriasis treatment Otezla, to include data from a scalp psoriasis study. This is a growth area for Amgen, with the global psoriasis market expected to reach $46.6 billion by 2022, according to a report by The Business Research Company. Amgen has nearly 40 drugs in the pipeline, and half are in phase 3 trials. If those late-stage trials lead to drug approvals, Amgen should be set for a new wave of growth in the future, even if there is a bit of a lull at the moment. And that's good news for long-term investors.

Finally, while Biogen doesn't pay a dividend, Amgen does. Amgen has steadily increased its dividend annually, and in 2019, the company paid an annual dividend of $5.80 per share, a 10% increase from the previous year.

Biogen or Amgen?

Biogen and Amgen face a slowdown in older drugs, but they have promising pipelines as well as newly marketed products that eventually will compensate for patent losses. In both cases, investors will have to be patient, as these new additions take time to enter the market and win over doctors and patients. Amgen shares have lost 17% since the start of the year, while Biogen has slipped less than 1%.

For me, that is the deciding factor. Considering long-term sales prospects, Amgen shares are depressed at the moment and offer a decent entry point. As for Biogen, it is likely the shares will see ups and downs from now until an FDA decision on aducanumab. Investors in biotech stocks may use share weakness to start a position in Biogen, though the most cautious may want to wait for more news on the Alzheimer's treatment before placing any bets.