It's been a great decade to own stocks in general, but those of you who bought Celgene Corporation (NASDAQ:CELG) or Biogen, Inc. (NASDAQ:BIIB) back in 2008 fared even better than most. Both big biotech stocks have more than tripled over the past 10 years.

Past performance doesn't mean they can do it again, though. In fact, both stocks have been beaten up in recent months. Let's look at the opportunities and challenges facing Biogen and Celgene and see how they stack up against each other. 

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The case for Celgene Corporation

Aging populations around the developed world are driving demand for cancer treatments through the roof. According to Evaluate Pharma, the overall size of the oncology market will swell at a 13% annual growth rate and hit $192 billion in 2022, and Celgene is well positioned to command a leading share.

Sales of Celgene's flagship blood cancer pill will play a big role in the spending increase. Revlimid's growing population of newly diagnosed multiple myeloma patients and steady price increases are expected to drive annual sales of the drug from $8.2 billion in 2017 to a whopping $15.2 billion in 2022.

The company's more recently launched drugs are also performing well. Management expects Pomalyst sales to jump 18% to $1.9 billion this year over last, and Otezla sales could grow 17% to $1.5 billion.

While Celgene's product lineup is firing on all cylinders now, important Revlimid patents expire in a few years. Despite contributions from more recently launched products, Revlimid sales still comprise 63% of total revenue, and the therapy will begin facing limited generic competition beginning in March 2022. 

Perhaps the most important experimental therapy in the company's late-stage pipeline hit an embarrassing road bump that followed an expensive disaster several months earlier. Despite the unexpected setbacks, Celgene still boasts a strong development pipeline, with five new drug candidates either wrapping up pivotal trials or waiting for a review. 

Earlier this year, Celgene acquired the portion of Juno Therapeutics it didn't already own. With full ownership of Juno's JCAR017, and a license to develop bb2121 with bluebird bio, Celgene also could also launch multiple new CAR-T therapies in the quarters ahead.

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The case for Biogen Inc.

During the 10-year period ended 2013, Biogen stock soared 1,120%, and it was the company's successful multiple sclerosis (MS) therapies that provided the lift. Although the company reported record revenue last year, competition in the MS space is heating up.

Biogen has three blockbuster MS drugs that generated around 81% of total revenue last year. Unfortunately, Tecfidera's the only one that grew sales last year. Luckily for Biogen, Roche (NASDAQOTH:RHHBY) licensed its recently launched MS therapy from Biogen years ago. 

Ocrevus is the first therapy strong enough to significantly slow down disease progression for patients with the most aggressive form of MS. That efficacy combined with a stellar safety profile made it the most impressive new drug launch of 2017. Ocrevus didn't launch until last April, but by the end of the year, it achieved $915 million in U.S. sales.

While Biogen's investors would prefer the company's wholly owned therapies lead the space, from now on the company's entitled to a 24% royalty on U.S. sales of Ocrevus. With Roche marketing the drug, those royalties are nearly all profit for Biogen.

Biogen isn't just a royalty taker; it's also a royalty payer. The company wisely licensed Spinraza from Ionis Pharmaceuticals (NASDAQ:IONS), and now it's leading Biogen's product lineup in sales growth. The spinal muscular atrophy treatment launched at the beginning of 2017 and before the end of the year racked up $884 million, $112 million of which went to Ionis.  

Biogen's most important experimental therapy is still in the middle of the most keenly watched pivotal trial of its time. Aducanumab put up strong data in an early Alzheimer's disease trial that informed an ongoing pivotal study the company rushed to initiate. If successful, aducanumab could become the first disease-modifying treatment for millions of Alzheimer's disease clamoring for a solution.

Even the industry's more conservative analysts think aducanumab can become a $10 billion-per-year drug if data from its ongoing pivotal studies fall in line with previous observations. While Biogen's candidate has the best shot so far, late-stage Alzheimer's trials have a 99% failure rate. 

The better buy?

At recent prices, Celgene looks like a bargain at a price around 10.4 times this year's earnings expectations, and so does Biogen at around 11.2 times forward estimates. While I think Biogen has more potential upside than Celgene, investors could suffer steep losses if aducanumab fails ongoing pivotal trials expected to read out in 2020.

With Biogen's uncertainty in mind, Celgene's predicament doesn't seem so bad. A more diverse pipeline means it will take more than one catastrophe to derail the company's long-term growth strategy. That makes Celgene the better buy right now.

Cory Renauer owns shares of Celgene. The Motley Fool owns shares of and recommends Bluebird Bio, Celgene, and Ionis Pharmaceuticals. The Motley Fool recommends Biogen. The Motley Fool has a disclosure policy.