Similarities between Merck & Co. Inc. (NYSE:MRK) and AbbVie Inc. (NYSE:ABBV) are easy to find. Both are big pharma companies with promising cancer drugs on the market. Both face some challenges with current products. Both pay attractive dividends.

For most of 2017, Merck stock outperformed AbbVie. However, AbbVie now claims higher year-to-date gains after a recent surge. Which of these drug stocks is the better pick for investors over the long run? Here are the arguments for Merck and for AbbVie. 

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The case for Merck

There's no better argument for buying Merck than the potential for Keytruda. Merck won approval from the U.S. Food and Drug Administration (FDA) for the drug in treating melanoma in September 2014, followed by approval in 2015 as a second-line treatment for non-small cell lung cancer (NSCLC). The drug continued to rack up approvals for other indications, perhaps most importantly getting a green light from the FDA in October 2016 as a first-line treatment for NSCLC and just last month for treating any solid tumor with a specific genetic feature known as microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR).

Wall Street analysts are so enthused about Keytruda's prospects that several of them peg the peak sales opportunity for the drug at close to $8 billion. This estimate assumes that late-stage clinical trials for Keytruda in treating other types of cancer go well.

Merck awaits approval for several drugs, most notably diabetes drug ertugliflozin as a stand-alone treatment and as part of two combinations. The company also has 11 late-stage programs in addition to Keytruda. 

The bad news for Merck is that sales are falling for several of its current products. In the first quarter of 2017, the drugmaker reported lower revenue from 20 of its top 33 drugs, with the biggest decline for cardiovascular drug Zetia.

Despite these headwinds, analysts think Merck will grow earnings by an average of 6% annually over the next few years, thanks primarily to Keytruda. Investors also should like Merck's dividend, which currently yields 2.91%.

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The case for AbbVie

AbbVie continues enjoy solid growth from its top-selling product Humira. Last year, Humira generated revenue of $16 billion, representing nearly 63% of the company's total revenue -- up 15% from the prior year.

Another drug is coming on strong, though. Sales for cancer drug Imbruvica soared 45% year over year to $551 million in the first quarter of 2017. AbbVie expects the drug to generate $5 billion annually by 2020. 

The company also has great expectations for another approved drug, Venclexta, which gained approval for treating chronic lymphocytic leukemia with 17p deletion in April 2016. Analysts have projected peak annual sales of up to $2 billion if the drug also wins approval for two other targeted indications, multiple myeloma and acute myeloid leukemia.

AbbVie is counting on its pipeline to deliver more blockbusters. On the immunology front, the company has two promising late-stage candidates in ABT-494 and risankizumab. In oncology, AbbVie should have big winners with Rova-T and veliparib.

With sustained momentum for Humira and Imbruvica combined with great potential for Venclexta and pipeline candidates, Wall Street analysts think that AbbVie can grow earnings by more than 14% annually on average over the next few years. Shareholders also benefit from the company's attractive dividend yield of 3.59%. 

Better buy

There are problematic areas for both of these companies. Merck's declining sales for several of its products put a lot of pressure on Keytruda to deliver as expected. Some potential safety issues in clinical studies evaluating the drug in treating multiple myeloma are concerning.  

AbbVie's primary challenge is the potential for biosimilar competition to Humira. Amgen won FDA approval for its Humira biosimilar last year but hasn't launched the drug yet due to a lawsuit filed by AbbVie. 

So which of these two drug stocks is the better pick? I'd go with AbbVie. I think the company will be able to hold off biosimilar rivals in the U.S. for at least four more years. That gives time for Imbruvica and Venclexta to fully ramp up and for new drugs from the pipeline to reach the market. AbbVie has some risks, but I like the stock's prospects over the new few years.