Aurora Cannabis (NYSE:ACB) and Scotts Miracle-Gro (NYSE:SMG) shareholders are probably glad that 2018 is behind them. Aurora's share price fell 35% last year -- although the steep drop stemmed entirely from dilution caused by issuing new shares -- while Scotts Miracle-Gro stock plunged nearly 43%.

It's a new year now, and both stocks have gotten off to a good start. 2019 should be a better year for both Aurora Cannabis and Scotts Miracle-Gro for several reasons. But which is the better pick for long-term investors?

The case for Aurora Cannabis

If you like simple investing theses, you'll probably love the investing thesis for Aurora Cannabis. It can be summarized in two statements:

1. Global demand for cannabis will soar.
2. As the producer with the greatest production capacity, Aurora Cannabis stock will soar, too.

I doubt that anyone would honestly argue against the first proposition. Supply isn't keeping up with demand in Canada's recreational marijuana market. And the country still hasn't finalized regulations for another potentially massive market -- cannabis edibles and concentrates.

Countries around the world have legalized medical marijuana -- Australia, Germany, and the United Kingdom, to name just a few. Mexico should soon follow Canada and Uruguay in legalizing recreational marijuana. Piper Jaffray analyst Michael Lavery recently predicted that the global cannabis market will top $250 billion annually and could reach $500 billion.

There's also no question that Aurora Cannabis isn't on course to rank as the top marijuana grower in terms of production capacity. With its flurry of acquisitions over the past couple of years, Aurora should be able to produce up to 700,000 kilograms of cannabis per year.

Of course, it takes more than just production capacity to be successful. Aurora also has a fast-growing global distribution network. Perhaps the only key ingredient that Aurora doesn't have that several of its rivals do is a big partner outside the cannabis industry. However, it could be just a matter of time before the company checks off that box, too.

Aurora's market cap currently stands at close to $6 billion. With the global cannabis market growing by leaps and bounds, the company should be able to achieve strong growth over the next decade if it executes well. 

The case for Scotts Miracle-Gro

Why buy Scotts Miracle-Gro stock? The reasons aren't too different from those for Aurora Cannabis. As the leading supplier of hydroponics and other products to the cannabis industry, Scotts should benefit from increasing cannabis demand. The company should also generate more modest growth with its core consumer lawn and garden products business.

There's one big advantage that Scotts Miracle-Gro has over Aurora right now: It can operate in the United States. That's a significant advantage, because the U.S. is the largest marijuana market in the world. Thirty-one states have legalized medical marijuana, with 10 states also legalizing recreational marijuana. And the marijuana markets in most of these states are still in their infancy.

The biggest market for Scotts' Hawthorne Gardening subsidiary, which caters to the U.S. cannabis industry, is California. In 2017, marijuana sales totaled around $3 billion. But by 2022, the figure is projected to soar to $7.7 billion. Even better, the potential for more large states to legalize recreational marijuana in the near future should dramatically boost the total addressable market for Hawthorne.

Scotts still makes more than 90% of its revenue, though, from its consumer lawn and garden products segment. The market for this segment is already mature and isn't going to produce the fast growth that the cannabis supplies market will. However, Scotts' introduction of new products should add incremental growth. Also, the potential for longer spring and summer seasons because of climate change could drive higher demand for the company's lawn and garden products.

In addition to its growth prospects, Scotts Miracle-Gro offers investors another nice perk. The company pays a dividend that currently yields nearly 3.6%.  

Better buy

Aggressive investors might find Aurora Cannabis more to their liking than Scotts Miracle-Gro. The company's growth opportunities should be greater. My main concern with Aurora is that it could continue issuing new shares to raise cash and dilute the value of existing shares. 

Scotts Miracle-Gro is the better pick for most investors, in my view. The stock's valuation is attractive with shares trading at 14 times expected earnings. Its dividend will help increase Scotts' total return over the long run. Scotts should also enjoy solid growth as marijuana sales increase in the U.S. and it launches new consumer lawn and garden products.

Aurora is likely to deliver great returns over the long run. But I suspect that the comparison between these two stocks is kind of like the tortoise-and-hare fable, with Scotts as the tortoise and Aurora as the hare. And we all remember who won that race.