If year-to-date performance were the deciding factor on which stock to buy, there would be no contest between Aphria (NASDAQOTH:APHQF) and MariMed(NASDAQOTH:MRMD). Aphria has lost nearly half of its market cap. MariMed ranked as the best-performing marijuana stock of the first half of 2018. Its share price is now up more than 260% so far this year.

But, as you've no doubt heard before, past performance is no guarantee of future results. Which of these two marijuana stocks is the better pick now? Here's how Aphria and MariMed compare.

The case for Aphria

Aphria is the fourth-largest Canadian marijuana grower in terms of market cap. It's also the only company in its peer group to have posted 11 consecutive quarters of positive adjusted EBITDA. But is Aphria worthy of investors' consideration now? Yes -- because of two key opportunities for the company.

The first opportunity is the recreational marijuana market in Canada, which is scheduled to open for business in October 2018. Aphria should be in great shape to become a leader in this market, just as it already is in the Canadian medical marijuana market.

Aphria's expansion efforts to add production capacity are moving along as planned. The company expects to grow 255,000 kilograms of cannabis annually next year. This capacity should work to its advantage as the domestic recreational marijuana market takes off. 

An even greater opportunity, however, is in the global medical marijuana market. Aphria's acquisition of Nuuvera earlier this year puts the company in a good position to compete internationally. The most important of these markets is Germany, which legalized medical marijuana last year. Aphria CEO Vic Neufeld recently expressed his confidence that the company will capitalize on the opportunity in Germany. 

Another plus for Aphria is its low production costs. This could be very important in the future when supply in the Canadian marijuana market catches up with demand. When that happens, Aphria should be in better shape than many marijuana growers to survive in a lower-price environment. 

The case for MariMed

MariMed is making its mark in the U.S. cannabis industry. The company provides professional management services to marijuana growers. These services range from helping customers obtain cannabis cultivation licenses to managing their growing facilities to providing legal and administrative services.

In addition, MariMed markets its own lineup of marijuana products. These cannabis-infused products include MariMints mints, Kalm-Corn popcorn, and Betty's Eddies fruit chews. 

The company currently operates six facilities in four states: Delaware, Illinois, Nevada, and Maryland. MariMed is also building two new facilities in Massachusetts that should be operational this year. Massachusetts is projected to have a marijuana market totaling more than $1 billion annually by 2022. 

Probably the biggest reason for investors to consider MariMed stock is the overall potential for growth in the U.S. marijuana market. This market could reach $22 billion by 2022. To put this number into perspective, it's more than three times the projected size in 2022 of the Canadian and German marijuana markets combined. 

Better marijuana stock

The decision between these two stocks isn't as clear-cut as it might seem. MariMed operates in the large U.S. market, while Aphria can't maintain its stock listing on the Toronto Stock Exchange and also have a significant U.S. presence as long as current federal marijuana laws remain in effect. However, Aphria should have a virtual lock on seeing tremendous revenue growth within the next few months thanks to Canada's legalization of recreational marijuana.

Although MariMed's current price-to-sales ratio of 68 is lower than Aphria's multiple of 76, I suspect that will look much different by next year. I think that Aphria has a clear path to growth in Canada over the next couple of years and pretty good prospects in international markets including Germany and the U.K.

There could also be some reversion to the mean, with Aphria stock rebounding from its big decline and MariMed giving up some of its huge gains. For these reasons, I think the nod goes to Aphria as the better pick for now.

Both companies could face significant challenges, though. Aphria will almost certainly have to deal with a supply glut in Canada within a few years. MariMed could find growth more challenging as the U.S. cannabis industry consolidates and larger players don't need the professional management services that smaller marijuana growers do.

Aphria and MariMed also both have huge growth baked into their share prices. Any bump -- or even the hint of one -- could cause the stocks to sink. Buying either of these marijuana stocks is a risky move suited only for the most aggressive investors.