Marijuana stocks have been some of the market's top performers this past year as investor optimism has soared. However, that impressive run might not carry over into 2015, at least not for all companies.

That's because 2015 could be the year marijuana investors become more selective, picking and choosing the best-in-breed plays and ignoring those with the most question marks. With that in mind, let's consider what might lie ahead in the new year for two of this year's biggest marijuana winners: GW Pharmaceuticals (NASDAQ:GWPH) and Insys Therapeutics (NASDAQ:INSY).

A bit of background

In 1996, California became the first state to approve the use of medical marijuana. An additional 22 states have since legalized marijuana for use in patient treatment, including Maryland, Minnesota, and New York in 2014.

Rising awareness of marijuana's potential as a therapy for pain caused by diseases such as cancer or glaucoma has sparked a wave of ballot measures legalizing marijuana possession for medical purposes. That awareness is also sparking renewed interest in legalizing marijuana for recreational use. So far, four states have passed laws allowing for possession of a small amount of recreational marijuana: Colorado, Washington, Oregon, and Alaska. Washington, D.C. has done the same.

The potential for additional states to follow up with their own cannabis legalization initiatives has some speculating that the legal sale of marijuana could provide billions of dollars per year in tax revenue. Marijuana theoretically could follow a similar path to tobacco tax revenue, which has climbed significantly in the past 20 years to more than $17 billion.

Marijuana also could mean big money for the companies that successfully scale cannabis production and distribution. To date, most companies that are attempting to profit from legal uses of marijuana are both small and unproven. As a result, they are traded as penny stocks in the over-the-counter, or pink-sheet, market. The OTC market is ripe for manipulation, and buyers should approach those companies very cautiously. However, it's not just pink-sheet marijuana stocks that have jumped this year. Two marijuana stocks that trade on the Nasdaq exchange are also surging. Those two biotechnology companies, GW Pharmaceuticals and Insys Therapeutics, are developing cannabinoid therapies that might someday be used to treat patients suffering from epilepsy, cancer pain, brain cancer, and possibly even diabetes and schizophrenia. The potential of these marijuana-derived medicines led to both companies' shares surging by about 64% in 2014.

Proving its concept

GW Pharma this year raised $212 million from secondary stock offerings, which will be used to fund development of Sativex, its THC-based therapy for multiple sclerosis and cancer pain, and Epidiolex, its CBD-based therapy for the treatment of epilepsy and other disorders.

Sativex is already on the European market as a treatment for MS spasticity, but the drug has only $6.8 million in sales during the past 12 months. GW Pharma hopes that winning FDA approval in 2015 of Sativex as a cancer pain therapy will change that. Cancer pain today is treated with opioids, but GW Pharma estimates opioids fail to control pain in 20% or more of cancer pain patients. The company plans to announce phase 3 results from its first cancer study early next year, with results from a second phase 3 study expected in the second quarter. If those results are positive, GW Pharma hopes to file for FDA approval in the second half of next year; with the agency's OK, Sativex could reach the U.S. market in 2016.

GW Pharma should also provide investors with more insight into its epilepsy programs in 2015. In October, the company kicked off Part A of its phase 2/3 trial in Dravet Syndrome, and Part B will begin in the first quarter. GW Pharma further plans to launch two phase 3 studies for Epidiolex in Lennox-Gastaut syndrome, or LGS, early next year. Most investors will focus on Sativex and Epidiolex, but they should also pay attention to GW Pharma's phase 2a trial of GWP42003 for use as a schizophrenia treatment. That trial should be complete in the second half of 2015.

Expanding its franchise

Similarly to GW Pharma, Insys has a full slate of catalysts that could move shares higher or lower next year.

However, unlike GW Pharma, which relies on stock offerings to raise money to fund its research, Insys already generates more than $200 million in annualized revenue from Subsys, an opioid medicine used to treat breakthrough cancer pain. Subsys' success means Insys has no debt and boasts a growing cash stockpile that is fueling its research and development programs without the need to dilute investors.

In 2015, Insys could win approval for a second therapy with nine-figure sales potential. Dronabinol, an oral formulation of the marijuana drug Marinol, could be submitted for FDA approval soon. Assuming it is approved, Insys believes the drug could capture a big chunk of the Marinol market, which is valued at $150 million and is growing by about 4% annually.

In the coming year, investors should also gain insight into Insys' early stage effort to develop its own CBD treatment for both Dravet and LGS. Insys plans to launch phase 3 trials in both indications before the end of next year. If the company can execute on that timeline, it could go head-to-head with GW Pharma in treating those indications.

One to buy

Both GW Pharma and Insys made big moves this past year, but Insys appears to be the better buy at this point. GW Pharma has an intriguing, but unproven, pipeline and a $1.3 billion market cap, while Insys has a top-selling therapy on the market, similar pipeline opportunities, an arguably better balance sheet, and a market cap of $1.5 billion that is only slightly higher than GW Pharma's. As a result, in the speculative portion of my portfolio, I'm going to watch and wait on GW Pharma and stay long on Insys in 2015.