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Two of the biggest biotechs have produced quite different results for shareholders. Amgen's (AMGN 1.56%) valuation nearly doubled during the the last five years, while Gilead Sciences' (GILD -1.21%) stock price more than quadrupled.

Looking only at performance this year is a different story altogether. Amgen has basically tread water, with shares close to the same price as at the beginning of 2016. Gilead's stock price, on the other hand, has dropped by a double-digit percentage year-to-date.

All of that is in the past, though. Which of these big biotech stocks is the better pick for long-term investors going forward? Let's see how Gilead and Amgen stack up against each other. 

The case for Gilead Sciences

Gilead rose to prominence on the strength of its HIV franchise. The company still claims a dominant position in the HIV drug market, with blockbusters like Truvada, Atripla, and Stribild. But while Gilead continues to make billions from its HIV drugs, hepatitis C drugs Harvoni and Sovaldi generate well over half of the biotech's total revenue.

Sales growth for the two hep C drugs has been nothing short of astounding. That growth has helped put Gilead in an enviable financial position: The company sits on a cash stockpile of over $8 billion. That amount was even higher, but Gilead has used some of its cash in ways that reward shareholders.

One important thing Gilead has done with its money has been to buy back shares. During the first quarter of 2016 alone, the company purchased $8 billion of its stock. Gilead's board authorized another $12 billion share buyback earlier this year.

Another shareholder-friendly move by the company was initiating a dividend in 2015. The dividend yield now tops 2% after a nice increase in February -- quite good for a biotech. 

Gilead also invested in some strategic deals. In April, the company announced plans to buy Nimbus Therapeutics' Acetyl-CoA Carboxylase (ACC) inhibitor program for treating non-alcoholic steatohepatitis (NASH) and other liver diseases. Gilead closed another deal in January to license anti-inflammatory drug filgotinib from Galapagos NV.

The bad news for Gilead is that the once-stellar earnings growth for Harvoni and Sovaldi has fizzled. Combined sales for the two drugs fell 6% year over year in the first quarter.

There is good news, though. Gilead recently won regulatory approval in the U.S. and in Europe for its latest hep C drug Epclusa, which is a combination of Sovaldi and velpatasvir. Epclusa is the first drug that treats all sub-types of hepatitis C. Analysts project that Epclusa could bring in up to $1.8 billion this year and reach peak annual sales of $10.3 billion in 2017, although it's likely that a significant portion of those sales will be at the expense of Harvoni and Sovaldi.

Gilead also has a strong pipeline with seven late-stage clinical trials in progress and another 17 phase 2 studies. Simtuzumab stands out as a bright prospect. The drug is in one phase 2 study targeting treatment of NASH and another targeting primary sclerosing cholangitis, an inflammatory disease that causes scarring within bile ducts. If approved, simtuzumab could ultimately hit peak annual sales of as much as $12 billion.  

The case for Amgen

Amgen claimed eight different blockbuster drugs in its lineup for 2015. Enbrel led the way, posting sales of nearly $5.4 billion last year. It's surely pleasing to Amgen that Enbrel is still going strong: First-quarter sales of the drug jumped 24%, year over year.

Neulasta isn't too far behind, either. The bone marrow stimulant brought in over $4.7 billion last year. Neulasta's sales continue to grow, but by relatively low percentage increases.

The biotech has at least a couple of rising stars in the mix. Sales for Prolia jumped 29% year over year in the first quarter of 2016. First-quarter sales of Kyprolis grew at an even faster rate of 43% compared to the prior-year period. 

Amgen does face some challenges. The U.S. patent for Neulasta expired last year, and the drug loses European patent protection in 2017. The biotech already faces competition from a biosimilar rival for Neulasta.

There are also plenty of opportunities for the big biotech. Amgen's pipeline includes 12 late-stage clinical trials with another nine phase 2 studies. The company has six biosimilars in development.

Those biosimilars might present some of Amgen's biggest potential. The company hopes to compete against powerhouse anti-inflammatory drug Humira with its ABP 501 biosimilar. ABP 798, a biosimilar for cancer drug Rituxan, is another possible winner.

And while Gilead's cash position looks good, Amgen's is even better. Amgen reported cash, cash equivalents, and marketable securities totaling over $34.7 billion at the end of the first quarter. The company also pays a dividend that currently yields just under 2.5%.

Better buy

Both of these biotech stocks boast great product lineups, strong pipelines, healthy financial statuses, and nice dividends. But there are two differences that jump out when you look closely at Amgen and Gilead.

Wall Street expects Amgen's growth prospects to be stronger over the next few years. That's not to say that the company will see awe-inspiring earnings growth. It likely won't, primarily because of the Neulasta headwinds. However, Gilead faces downward pricing pressure on its hepatitis C franchise that seems likely to dampen the biotech's growth prospects.

The other significant difference between these two stocks, though, is valuation. Amgen trades at a relatively attractive forward earnings multiple of 13.36. But that's a lot higher than Gilead's stupid cheap forward earnings multiple of 6.95.  

I like Amgen. But if I could only own one of these stocks, I'd go with Gilead. In my view, Gilead is a bargain at its current price. Maybe the company won't have huge earnings growth for the next few years, but it should keep earnings roughly at current levels or better.

The current level of earnings, by the way, amounted to $18.1 billion last year. That kind of money can fund plenty of share buybacks, dividend increases, and strategic deals. I expect Gilead to do all of those things, driving shares higher. I also think that the bold new frontier for the biotech will probably be in the NASH indication. Don't be surprised if Gilead's earnings growth turns out better than expected, thanks to success in this area.