Biogen Inc. (NASDAQ:BIIB) and Gilead Sciences, Inc. (NASDAQ:GILD) were two of biotech's best-performing stocks in years past, but investors are a lot more worried about where they're going right now. Gilead Sciences has a new management team and a new focus on treating cancer. Now that competitors have diminished Biogen's dominance in the multiple sclerosis space, the company's taking bold steps as well.

Investors are right to wonder which of these two are the better stock to buy now. Let's stack them side by side to see which stands taller.

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The case for Gilead Sciences, Inc.

If you're looking for biotech stocks that won't keep you up at night, Gilead Sciences fits the bill. Sinking sales of the company's hepatitis C antiviral (HCV) treatments incited investors to push the stock down to just 11 times this year's earnings expectations, at a time when the average stock in the benchmark S&P 500 index trades at 17.8 times earnings expectations.

Gilead Sciences had to cut prices for its HCV drugs in response to fierce competition from AbbVie's Mayvret. HCV sales plummeted to just $1 billion in the second quarter from $2.9 billion a year earlier. On the bright side, HCV sales comprise just 18% of total revenue now, which gives new drug launches in HIV and oncology a chance to tilt Gilead's top line upward again.

Biktarvy is the first and only all-in-one pill for HIV patients that are new to treatment or replacing an old one, and some analysts think it could add more than $5 billion to Gilead's annual top line at its peak. Gilead's blockbuster-to-be achieved an annualized run rate of $740 million during the three months ended June, despite launching in February. 

The first drug to emerge from Gilead's $11.9 billion acquisition of Kite Pharma last year isn't a simple once-a-day pill. Yescarta's essentially an IV bag full of a lymphoma patient's own immune cells that have been modified off-site to recognize and fight cancer cells once they've been redeployed. 

Yescarta launched in late 2017, and sales hit an annualized $272 million run rate in the second quarter, which isn't bad considering it's limited to patients with a rare lymphoma who have relapsed after two other lines of therapy. A recent approval in the EU could give Yescarta's launch some extra lift in the quarters ahead.

Although HCV sales have fallen for Gilead Sciences, the company still generates a strong profit that's allowed it pay a dividend that offers a 3.2% yield at recent prices. The company used just 34% of free cash flow generated over the past year to meet its dividend commitment, which gives Gilead plenty of room to make large increases in the years ahead. 

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The case for Biogen Inc.

Biogen doesn't pay a dividend, but its market-leading multiple sclerosis treatments have helped this biotech return a lot of cash to investors in the form of buybacks. In the first half of 2018, Biogen lowered its outstanding share count by around 5%, with a whopping $3 billion in share repurchases.

Biogen could have a lot more cash to return to investors in the years to come if some high-risk neurology bets pay off. The company started a massive trial with its lead Alzheimer's candidate, aducanumab, in 2015, and we could have top-line results next year. if it truly slows progression of the disease, it could become one of the best-selling drugs of its time.

A collaboration with RNA-based drug developer Ionis Pharmaceuticals (NASDAQ:IONS) added Spinraza to Biogen's product lineup at the end of 2016. Sales of the rare-disease drug are on pace to hit $1.7 billion this year, but Novartis has a candidate in late-stage development that could force Biogen to offer insurers deep discounts within another couple of years.

About 61% of Biogen's total revenue comes from three multiple sclerosis drugs, and none of them are looking good right now. In the second quarter, Biogen reported declining year-to-year sales for Tecfidera, Avonex, and Tysabri. None are in freefall, but it looks as if their best years are in the past.

The better buy

There isn't any gnashing of teeth necessary to choose the better buy among this pair. Biogen's neurology bets could make shareholders very rich, but the odds are long and the risks are hard to ignore. A flop for aducanumab and success for Novartis could easily lead to heavy losses down the line.

Gilead Sciences has a stronger pipeline, a lead growth driver with a long life ahead of it, and a more attractive valuation. That makes it the better buy today, hands down.

Cory Renauer owns shares of Gilead Sciences. The Motley Fool owns shares of and recommends Biogen, Gilead Sciences, and Ionis Pharmaceuticals. The Motley Fool has a disclosure policy.