After a booming 2017, most marijuana stocks aren't performing as well so far in 2018. The biggest marijuana stock, Canopy Growth Corporation (NYSE:CGC), is up by only a single-digit percentage. Shares of another major Canadian marijuana grower, Cronos Group (NASDAQ:CRON), are down a little year to date.

But major changes are on the horizon for both Canopy Growth and Cronos. Which of these marijuana stocks is the better buy? Here's how Canopy and Cronos compare.

The case for Canopy Growth

Investors have flocked to the stocks of Canadian marijuana growers for two primary reasons. One is the rapid growth in the global medical marijuana market. The other is the anticipated explosion of demand if Canada legalizes recreational marijuana. Canopy Growth is in great position to capitalize in both areas.

Canopy already claims the No. 1 spot for sales of medical marijuana in Canada. The company reported revenue of CA$21.7 million in its latest quarterly results, a 123% year-over-year increase. That wasn't just a record high for Canopy Growth; it was also the all-time high sales level for any marijuana grower in the country.

But Canopy hasn't just set its sights on the domestic medical marijuana market. Germany is quickly becoming an important source of revenue. Canopy also has operations or partners in several other countries that allow legal use of medical marijuana, including Australia, Brazil, Chile, Denmark, Jamaica, and Spain.

Planned legalization of recreational marijuana in Canada presents a huge opportunity for Canopy Growth. The company has scaled up its production capacity to be able to take advantage of what most expect to be very high demand for recreational marijuana. Canopy is building new facilities and is on track to have over 5.6 million square feet of domestic growing space.

Canopy Growth received what amounted to a tremendous endorsement last October, when Constellation Brands (NYSE:STZ) bought a 9.9% stake in the company. Constellation is one of the world's largest alcoholic-beverage makers, with products including Corona beer and Casa Noble tequila. This deal brought in a large infusion of cash. It also positions Canopy nicely in the cannabis-infused beer market.

The case for Cronos

The opportunities for Canopy Growth also largely apply to Cronos Group. Like Canopy, Cronos has global medical marijuana and Canadian recreational marijuana markets in its cross-hairs.

Cronos is much smaller than Canopy, though. For the quarter ended Sept. 30, 2017, Cronos reported sales of CA$1.3 million -- up a whopping 962% from the prior-year period and more than double the total from the previous quarter.

Germany could become the next big medical marijuana market for Cronos. The company signed an exclusive supply agreement with Pohl-Boskamp, a pharmaceutical manufacturer and supplier that distributes products to more than 12,000 German pharmacies.

Cronos is also ramping up its production capacity to be in position to meet demand when Canada legalizes recreational marijuana. The company is expanding its Peace Naturals facility to add 286,000 square feet of indoor production space and a 1,200-square-foot extraction lab. Cronos projects that its domestic annual production capacity will top 40,000 kilograms by 2019.

The company also recently became the first Canadian marijuana grower to list its stock on the Nasdaq stock exchange, a move that exposed Cronos to more U.S. investors. Canopy Growth had reportedly hoped to be the first of its peers to list on the Nasdaq, but the Constellation deal sidetracked it.

Better buy

Which of these two marijuana stocks is the better choice for investors? My nod goes to Canopy Growth.

One key decision criterion for me is capacity. Canopy Growth clearly has significantly greater capacity than Cronos does, which means the company should be able to capture more of the recreational marijuana market in Canada.

Another important factor, in my view, is Canopy's international presence. Although Cronos is targeting some of the same international markets, Canopy Growth has a head start.

I also think Canopy Growth's relationship with Constellation Brands is a major plus. It's too soon to know how big the cannabis-infused beer market will be, but the two companies could enjoy considerable success.

Canopy Growth isn't without its share of risks, though. The market size for recreational marijuana in Canada and the global medical marijuana market might not be as large as some predict. If demand is too low, there could be a supply glut that hurts Canopy Growth stock -- as well as the stocks of other marijuana growers. I think, however, that Canopy should enjoy nice gains over the next couple of years as the market expands.