AbbVie (ABBV 0.69%) and Gilead Sciences (GILD -1.29%) are names you might be hearing a lot these days. AbbVie is in the process of acquiring Botox maker Allergan (AGN) to boost its growth potential and reduce its reliance on the world's current top-selling drug, Humira. Meanwhile, Gilead's investigational coronavirus treatment, remdesivir, is in clinical studies, putting that company in the spotlight.

So far this year, optimism about finding a coronavirus treatment has been the wind in Gilead's sails, lifting the stock 24%. Over the same time, AbbVie shares fell by 7%. I like these stocks and would be happy to add both of them to my portfolio. But if I had to choose just one ... well, let's have a closer look and see which offers the best buying opportunity right now.

A top-down view of a table with "M&A" in big, block letters and hands writing in notebooks and typing on keyboards

Image source: Getty Images.

The case for AbbVie

Blockbuster anti-inflammatory drug Humira, with $19 billion in annual sales, accounts for 60% of AbbVie's annual revenue. The drug's sales are still growing in the U.S., but they have slowed in Europe after biosimilars entered the market. Humira, used to treat nine autoimmune conditions, may face biosimilar competition in the U.S. as of 2023, when patents expire.

That's exactly why AbbVie's acquisition of Allergan is great news. Humira will represent less than 40% of sales at the new AbbVie. And the addition of Allergan's portfolio lifts AbbVie's plans for annual growth, with sales of more than $30 billion in 2020 -- excluding Humira -- and expectations for a high single-digit annual growth rate.  

The deal includes antipsychotic drug Vraylar as part of a $1.2 billion neuroscience program. The potential blockbuster posted a 76% gain in net revenue last year to reach $857.5 million. AbbVie also gains gastrointestinal, women's health, eye care, and Botox therapeutics platforms. Medical aesthetics such as Botox and Juvederm are part of the agreement as well, representing $4.3 billion. Those assets will become part of a new business, Allergan Aesthetics, to be operated under the AbbVie umbrella.

AbbVie and Allergan have met conditions required for the transaction to move forward in Europe and are now awaiting approval from the Federal Trade Commission in the United States. AbbVie expects a decision early in the second quarter. Once the deal has closed, we can be optimistic about good days ahead. AbbVie has said the combination will add 10% growth to adjusted earnings per share in the first full year.

The case for Gilead

Gilead is the star of the moment, with hopes of remdesivir winning the coronavirus treatment race driving the shares higher. That trend may continue if positive news emerges from the trials. But there is more to this company than its work to treat COVID-19, the illness caused by the novel coronavirus.

Gilead offers a growing HIV program in a global market that is forecast to reach $40.7 billion by the end of 2026, according to Fortune Business Insights. All of Gilead's HIV drugs together posted a 12% gain in sales to reach $16.4 billion last year. Sales of Gilead's blockbuster HIV drug, Biktarvy, quadrupled to $4.7 billion last year from the prior year's level, and EvaluatePharma predicts that number may reach almost $7 billion in 2024.

On the horizon, Gilead may have another growth driver. The company submitted filgotinib, its rheumatoid arthritis drug, to the U.S. Food and Drug Administration in December. With the drug under priority review, a decision may come as soon as June. Gilead said during its fourth-quarter earnings call that the drug has the potential to launch in five new indications over the coming four years.

Also on the earnings call, the company pledged to bring 10 "transformative" treatments to patients over the next decade. That is more good news, especially considering the decline in revenue from Gilead's chronic hepatitis C program.

Which stock to choose?

Though Gilead shares are up so far this year, there's likely more to come through the revenue drivers mentioned above. As for AbbVie, the Allergan purchase offers reason to be optimistic about future share performance.

When it comes to revenue, AbbVie may be a few steps ahead of Gilead. The acquisition of Allergan offers an exciting boost in growth for a company where revenue has already seen steady gains since the spinoff from Abbott Laboratories (ABT 1.96%) in 2013. At Gilead, sales only picked up last year after declining from 2016 through 2018. Through the Allergan acquisition, AbbVie clearly has a quicker path to growth.

AbbVie also beats Gilead when it comes to valuation. AbbVie stock trades at close to 16 times earnings, its cheapest by that measure since 2016. Gilead is down from past highs, but its price-to-earnings ratio has climbed from less than 15 earlier in the year to reach 20 now.

I think AbbVie and Gilead are both good investments, but if I had to choose just one to buy right now, I would go with AbbVie.