Image source: Gilead Sciences.

For much of the past decade, Biogen (NASDAQ:BIIB) ruled as perhaps the best big biotech stock around. That changed over the last year, though, as Biogen's stock dropped like a rock. If you look at best stock performance over the past 10 years among big biotechs, Gilead Sciences (NASDAQ:GILD) now ranks at the top. But which of these biotechs is the better buy now for long-term investors?

The case for Gilead Sciences
Few biotechs are priced as attractively as Gilead is right now. The stock trades at an earnings multiple below eight for both trailing-12-month earnings and forward earnings. 

Gilead reported sizzling 31% year-over-year revenue growth in 2015. Earnings soared nearly 50% compared to the previous year. Much of that improvement stemmed from Gilead's phenomenally successful hepatitis C drug Harvoni.

To put things in perspective, Harvoni became a blockbuster drug in just a few months at the end of 2014. In its first full year on the market, the drug racked up sales topping $13.8 billion. Gilead's hep C lineup, consisting of Harvoni and its predecessor Sovaldi, made a combined $19.1 billion in revenue during 2015.

And that's not all. Gilead first made its name in the HIV/AIDS therapy market. The biotech's HIV franchise generated sales of almost $11 billion last year. Meanwhile, non-antiviral products kicked in another $1.9 billion in 2015.

As if all that weren't enough to get investors excited, Gilead also is one of the few biotechs that pays a dividend, sporting a forward yield around 2%. As you might expect for a company making over $18 billion in profits, Gilead boasts a low dividend payout ratio. Low actually might be an understatement, actually, for a payout ratio of under 11%.

What's the bad news? Harvoni has competition. Merck (NYSE:MRK) recently introduced its own hep C drug, Zepatier, with pricing well below Harvoni's list price. To add insult to injury, Merck also won a court decision that requires Gilead to pay $200 million for patent infringement. Another decision could ultimately force Gilead to pay royalties to Merck on all future sales of Harvoni and Sovaldi.

The case for Biogen
Biogen also can claim an attractive valuation, albeit not to the degree that Gilead can. Biogen's forward earnings multiple of 12.5 is well below that of the S&P 500. 

The big biotech reported a solid 2015, with revenue up 11% year over year. Earnings grew 21% compared to the prior year. As with Gilead, one drug led the way for Biogen.

Sales for multiple sclerosis (MS) drug Tecfidera increased 25% to $3.6 billion in 2015. Biogen also saw impressive growth from another MS drug: Revenue for Plegridy jumped to $338.5 million from $44.5 million in 2014.

Biogen experienced success with its hemophilia lineup as well. Sales for Aprolix climbed 67% year over year, while sales for Eloctate more than quadrupled. Combined, the two drugs generated over $554 million last year.

Looking toward the future, Biogen has five phase 3 clinical studies in progress and one drug awaiting regulatory approval (daclizumab). Perhaps the biggest potential winner in the biotech's pipeline is aducanumab, which targets treatment of Alzheimer's disease. Clinical studies thus far for the drug have produced both excitement and disappointment. If aducanumab does ultimately gain approval in this difficult indication, it could be a game changer for Biogen.

What's the downside for Biogen? Revenue from two of the company's blockbuster MS drugs, Avonex and Tysabri, is declining. There's also considerable risk associated with Biogen's pipeline, especially for aducanumab. 

Better buy
In my view, either of these biotech stocks would be good picks. If I had to pick just one, though, I'd go with Gilead Sciences. 

Gilead is trading at a valuation well below that of its peers. That's mainly because of concerns about Merck gaining market share with Zepatier, but so far those fears seem to be overblown. And Gilead has another likely hep C winner on the way: A combo treatment for all types of hepatitis C awaits regulatory approval in the U.S. and Europe. In addition, Gilead has a couple of HIV drugs and a hepatitis B drug that could hit the market relatively soon if all goes well with regulatory reviews.

On top of all that, Gilead has eight other late-stage studies in progress. What's especially encouraging for the biotech's long-term outlook is that five of those are in areas outside of Gilead's traditional antiviral focus.

I like Biogen's MS franchise and pipeline potential. However, I really like Gilead's valuation, hep C and HIV franchises, pipeline potential, dividend, and (not to leave anything out) its enormous cash stockpile. Better buy? Definitely Gilead Sciences.