Big pharma stocks are a prized commodity among growth and income investors alike. The core reasons boil down to the fact that pharmaceuticals are an essential staple for millions of people, and the industry has been a hotbed of innovation over the past decade.

Nonetheless, pharmaceutical stocks do have a serious dark side. Patent expirations for top-selling products, unexpected competition, clinical and regulatory setbacks, and political headwinds are just a few of the reasons behind the absolutely hair-raising level of volatility within this group of equities. Pharmaceutical stocks, in fact, have consistently been one of the most volatile cohorts within the entire market since the early 2000s. 

Wooden blocks with various healthcare symbols being stacked into a pyramid on a wooden table.

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Which big pharma stocks are worth this gut-wrenching roller coaster ride? AbbVie (NYSE:ABBV) and GlaxoSmithKline (NYSE:GSK) are two of the industry's most trusted names among value, growth, and income investors, thanks to their top-flight product portfolios and generous dividend programs. Which of these pharma giants is the better buy right now? Let's dig deeper to find out. 

The case for AbbVie

Illinois-based AbbVie began life in 2013. The drugmaker was originally spun off from healthcare giant Abbott Laboratories (NYSE:ABT) in a move designed to unlock the latent value in both underlying businesses. Post-split, Abbott became a medical products specialist, while AbbVie inherited the company's immunology-heavy pharmaceutical portfolio and pipeline.

This strategic decision has proved to be a stroke of genius in many ways. Abbott and AbbVie have both gone on to produce market-crushing returns for shareholders over this period, as well as two of the most highly coveted dividend programs. Both companies, in fact, are included in the esteemed list of S&P 500 Dividend Aristocrats. Nonetheless, AbbVie was born with a hefty burden to bear -- namely, the eventual loss of exclusivity for top-selling immunology medicine Humira.

Since 2013, Humira has gone on to become the world's best-selling medicine, raking in approximately $20 billion in global sales in 2018. The downside is that Humira's rapid growth has kept AbbVie from truly diversifying its revenue stream. Despite spending $21 billion to acquire the megablockbuster blood cancer drug Imbruvica in 2015 and organically developing multiple high-value drugs of its own over the years, Humira still makes up nearly 60% of the drugmaker's annual revenue stream. 

This year, AbbVie's management decided it was high-time to go all out to deal with its overreliance on Humira. To do so, the drugmaker joined forces with Botox maker Allergan (NYSE:AGN) in one of the largest mergers in biopharma history.

Before this merger announcement, Allergan was in a tailspin because of a host of managerial missteps, combined with the negative impacts stemming from the battle over the patent expiration for eye medication Restasis. AbbVie, in turn, apparently saw Allergan's misfortunes as the perfect opportunity to deal with the Humira problem. The two companies are now set to consummate their merger in the first quarter of 2020

What's in store for this pharmaceutical supergiant in 2020 and beyond? The combined entity will sport best-in-class product portfolios in CNS, immunology, medical aesthetics, and women's health products. The flip side is that the new AbbVie will also happen to be one of the most highly leveraged biopharma companies in the world.

To deal with this debt issue, AbbVie plans on using Humira's sizable revenue stream to delever as fast as possible. Fortunately, the drugmaker does have a solid three years before Humira loses exclusivity in the states, giving it a nice cushion to iron out the kinks. AbbVie also scored two major regulatory approvals in immunology this year with Skyrizi and Rinvoq, which should further this deleveraging effort in a big way.  

The case for Glaxo

Glaxo, in its current iteration, is about to turn 19 years old next month. The British multinational pharmaceutical giant, however, is about to undergo a radical transformation that will see it split into a consumer healthcare business and a standalone pharmaceutical/vaccines company in the not-so-distant future. This forthcoming split is the result of Glaxo's consumer healthcare joint-venture with fellow pharma giant Pfizer (NYSE:PFE).   

What can investors expect from Glaxo's growth-oriented pharma and vaccines business in the next decade? Glaxo has had amazing success in the vaccine space of late, fueled by the skyrocketing sales of shingles vaccine Shingrix. However, Glaxo is still attempting to regain a foothold in the high-growth oncology arena.

After selling off its oncology assets in 2014, Glaxo stormed back into the space with the $5.1 billion acquisition of Tesaro for the PARP inhibitor Zejula last year, and the company has also tacked on several high-value clinical candidates in oncology as well. Oncology, in turn, should prove to be a key component of the company's overall growth profile in the coming decade.  

Apart from vaccines and cancer meds, Glaxo's core respiratory franchise has also returned to its winning ways of late. Newer respiratory meds like Trelegy and Nucala have quickly come into their own as strong growth products, which has helped to offset the declines emanating from the former star asthma medication Advair. Glaxo's HIV joint venture ViiV Healthcare has been a surprising bright spot as well, even though it has had to compete against the 800-pound gorilla that is Gilead Sciences.  

Which stock the better buy? 

Although Glaxo has absolutely trounced AbbVie in terms of share price performance in 2019, this trend should flip in 2020, perhaps dramatically so. That's not to say Glaxo won't turn out to be a winning play next year, but AbbVie simply has more momentum heading into the new year.

The back-to-back approvals of Skyrizi and Rinvoq -- in conjunction with this game-changing Allergan merger -- have markedly changed the sentiment around AbbVie's stock. Moreover, AbbVie's management has made a clear commitment to the company's dividend program, despite its junk bond-like yield of 5.46%. Glaxo's brain trust, on the other hand, hasn't made any long-term commitments in regard to maintaining -- much less growing -- the biopharma's enormous annualized yield of 4.4%.

AbbVie, in kind, stands out as the more attractive income and growth play than Glaxo, although both stocks could easily end up beating the broader markets in 2020.   

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.