Big pharma is on a roll.

Several of the largest drugmakers' stocks are up by double-digit percentages over the past six months. Count AbbVie (ABBV 0.98%) and Pfizer (PFE 0.23%) among big pharma's best performers in 2016: AbbVie's shares are up over 13% since March, while Pfizer stock has climbed over 16%.

Can these two pharmaceutical companies keep up their winning ways? And is one stock a better pick than the other? Let's look at how AbbVie and Pfizer stack up.

The case for Pfizer

The argument for buying Pfizer stock right now can be summed up with two mathematical words: addition and division.

Pfizer has been adding to its business through strategic acquisitions. Since last September, the big drugmaker has completed a buyout of Hospira, bought two vaccines from GlaxoSmithKline, acquired Anacor Pharmaceuticals, and announced plans to acquire Medivation and purchase AstraZeneca's late-stage small-molecule anti-infectives business. If it were not for a decision by the U.S. Department of Treasury that prevented Pfizer from reducing taxes by moving its domicile to Ireland, we could add Allergan to the list.

If it sounds like Pfizer is trying to buy its way to growth, it's because that's exactly what the company is attempting to do. The Hospira deal kicked in $1.1 billion in additional revenue in the second quarter alone. Look for Medivation's Xtandi to make a difference for Pfizer's financials as soon as the acquisition closes and for Anacor's crisaborole to contribute to the coffers in the near future, assuming the drug gains regulatory approval.

These acquisitions aren't without criticism. Pfizer paid $14 billion for Medivation and $5.2 billion for Anacor. Skeptics say the company shelled out too much. However, Pfizer thinks the growth prospects for Xtandi and crisaborole, combined with Medivation's and Anacor's pipelines, justify the deals. I view the acquisitions as good fits with Pfizer's business. Medivation's products help Pfizer build up a stronger oncology portfolio along with Ibrance, while Anacor's crisaborole  complements Pfizer's efforts in the inflammation and immunology space.

What about the second element -- division? Pfizer announced that it's considering splitting its innovative health business, which includes drugs like cancer drug Ibrance and rheumatoid arthritis drug Xeljanz, from its established products business. Sales for Ibrance, Xeljanz, and some of the other drugs in the innovative health business are growing at a solid pace, while sales for many of Pfizer's established products are declining. The company expects to make a final decision before the end of 2016.

The last time Pfizer took the division route, it went very well for investors. In 2013, the company spun off its animal health business. Shares of the newly formed entity, Zoetis, have climbed over 65% since then, achieving more than double Pfizer's stock gains.

Pfizer is also rewarding investors in another way. The drugmaker returned around $8.7 billion to shareholders in the first half of this year through share buybacks and dividend payments. 

The case for AbbVie

While Pfizer is adding and dividing its way past patent-cliff woes, AbbVie is hoping to forestall its own patent cliff. Humira ranked as the world's top-selling drug in 2015, but the main patent for the biologic expires at the end of this year. However, AbbVie has won multiple ancillary patents in efforts to keep biosimilar rivals off the market until 2022 -- and maybe longer.

Sooner or later, of course, Humira will face competition. AbbVie plans to be ready for that day with an arsenal of other drugs. Leading the way is hepatitis C drug Viekira, which generated $833 million in revenue in the first half of this year. Imbruvica wasn't far behind, raking in $820 million in the same time period. Imbruvica should emerge as the bigger winner eventually. Analysts expect the drug to reach peak annual sales of around $7 billion for AbbVie, with additional revenue going to the company's partner Johnson & Johnson.

Blood cancer drug Venclexta could also be a big story for AbbVie. The drug, which won FDA approval in April, is projected to hit peak annual sales of $1.7 billion (although AbbVie has to share revenue with Roche). Also, a successor to Viekira is in late-stage clinical development. AbbVie hopes to have the combination treatment of ABT-530 and ABT-493, a one-dose treatment for all genotypes of hepatitis C, on the market next year.

AbbVie isn't depending only on organic growth. It gained Imbruvica from the 2015 buyout of Pharmacyclics. In June, AbbVie completed the acquisition of Stemcentrx, picking up promising cancer drug rovalpituzumab tesirine (Rova-T) in the process.

Like Pfizer, AbbVie is no slouch when it comes to dividends and share buybacks. The company's dividend yield currently stands at an attractive 3.53%. In June, AbbVie announced plans to repurchase $3.8 billion of its stock. 

Better buy

Which stock is the better choice? If I had to make the decision right now, I'd go with AbbVie. Humira continues to rock along and should do so for several years to come. The rest of AbbVie's product lineup seem well-positioned to make significant contributions as well.

I generally like Pfizer's buying spree, although the company might have paid out more than it should have. The big drugmaker had to do something to jump-start growth -- and it did. I'm also eager to find out Pfizer's plans for a potential separation into two companies. My hunch is that such a move would really generate investor excitement for the innovative products business. Should Pfizer announce another spinoff, I might have to reconsider my pick of AbbVie as the better buy.