If you're looking to invest in big biotech stocks, two top candidates that you'll probably want to consider are Biogen (NASDAQ:BIIB) and Amgen (NASDAQ:AMGN). Amgen is by far the larger of the two biotechs, with a market cap roughly double that of Biogen. And Amgen has handily outperformed Biogen so far this year.

But size and past performance simply don't mean much when you're trying to determine which stock will pay off the most over the long run. Which of these two biotech stocks is the better pick now? Here's how Biogen and Amgen compare.

Two scientists standing side by side in a lab.

Image source: Getty Images.

The case for Biogen

Biogen is best known for its powerhouse multiple sclerosis (MS) franchise. Tecfidera dominates in MS and is on track to generate sales of around $4.2 billion this year. Biogen's interferon products, Avonex and Plegridy, should rake in close to $2.4 billion in 2018. Tysabri shouldn't be too far behind with sales in the neighborhood of $1.9 billion. Fampyra and Zinbryta round out Biogen's MS lineup with much lower sales.

The downside to Biogen's MS franchise, though, is that their overall sales are slipping. While sales for Tecfidera have risen slightly this year, it hasn't been enough to offset the declines for most of Biogen's other MS drugs.

But there's good news on another front. Spinal muscular atrophy (SMA) drug Spinraza is performing very well. It's Biogen's latest blockbuster and was the primary driver for the biotech's double-digit revenue growth in the third quarter

Biogen does have three other products in its current lineup -- psoriasis drug Fumaderm, Enbrel biosimilar Benepali, and Remicade biosimilar Flixabi. However, these products don't amount to much more than an asterisk in Biogen's overall financial results right now.

Then there's Biogen's pipeline. The biotech is evaluating aducanumab in late-stage clinical studies for treating Alzheimer's disease. If successful, the drug could potentially become one of the top-selling drugs in the world. In addition, Biogen has another late-stage Alzheimer's candidate with elenbecestat. 

The company hopes to boost its MS franchise with BIIB098, which it licensed from Alkermes. Biogen also kicked off a phase 3 study in September for BIIB093 in preventing and treating severe cerebral edema in large hemispheric infarction (LHI), one of the most severe types of ischemic stroke. 

The case for Amgen

Amgen's current roster includes half a dozen blockbuster drugs. Autoimmune disease drug Enbrel is the biotech's biggest winner, with likely 2018 sales of close to $5 billion. Neulasta, which stimulates the growth of white blood cells following chemotherapy, should pull in over $4 billion this year.

Osteoporosis drug Prolia continues to enjoy solid momentum and is on track to generate revenue of well above $2 billion in 2018. Chronic kidney disease anemia drug Aranesp, osteoporosis drug Xgeva, and secondary hyperparathyroidism drug Sensipar should each make around $1.8 billion this year.

The problem for Amgen is that several of these blockbuster drugs are getting long in the tooth. As a result, the biotech's revenue growth in Q3 was sluggish. However, Amgen has several up-and-coming products that could make more of a difference in the future.

Migraine drug Aimovig deserves to be at the top of the list. Amgen still has high expectations for its cholesterol drug Repatha. Multiple myeloma drug Kyprolis continues to pick up momentum. 

Amgen's pipeline includes eight late-stage programs. Four of these programs seek additional indications for already-approved drugs. The biotech awaits approval for one of the other candidates, osteoporosis drug Evenity. Like Biogen, Amgen has a late-stage drug targeting treatment of Alzheimer's disease, AMG520. Perhaps the most promising candidate, though, is asthma drug tezepelumab.  

Most biotechs don't pay dividends, but Amgen does. Amgen's dividend currently yields more than 3%. And the company has increased its dividend payout by 138% over the last five years.

Better buy

I think that the safer bet between these two biotech stocks is Amgen. Although the company faces headwinds for its aging blockbusters, Amgen has a huge cash stockpile and a solid cash flow that's not going to disappear anytime soon. I have said in the past that Amgen could be a boring dividend stock over the next few years, but that's not necessarily a bad thing.

Biogen, though, will likely appeal to more aggressive investors. The biotech has its own challenges with its MS franchise. Spinraza could see competition in the not-too-distant future. However, Alzheimer's disease drug aducanumab could be a game changer for Biogen. Keep in mind that quite a few once-promising Alzheimer's treatments have crashed and burned. Still, Biogen stock could skyrocket with positive phase 3 studies for aducanumab.

Take your pick between these two biotech stocks based on your investing style. I'm personally not enamored of either Biogen or Amgen, though. In my view, there are other biotech stocks that provide better growth prospects with less risk.

Keith Speights has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Alkermes and Biogen. The Motley Fool recommends Amgen. The Motley Fool has a disclosure policy.