Moderna (MRNA -0.15%) and Axsome Therapeutics (AXSM 3.30%) are early in their revenue stories. Moderna launched its first product -- the COVID-19 vaccine -- in 2020. Axsome started selling its first two products -- a sleep disorder drug and an antidepressant -- this year.

Both stocks have delivered great returns to early investors. In 2020, Moderna shares soared more than 400%. This year, Axsome has gained about 100%. These exciting biotech companies each have late-stage candidates in the pipeline and offer promising futures. But which one represents the best buy today? Let's find out.

The case for Moderna

Moderna's share price performance disappointed this year. The stock is heading for a 25% loss. That's because investors have worried about Moderna's earnings prospects once the pandemic is over.

In recent months, though, Moderna offered us some clues about the post-pandemic revenue picture. And it looks bright. The global market may resemble that of the flu vaccine market. That means it could be a revenue opportunity of $12 billion to $24 billion.

Of course, Moderna would share the market with other vaccine makers. But there still is plenty of potential for its coronavirus booster to generate blockbuster revenue.

But here's even better news. Moderna probably won't be a one-product company for long. The biotech has 48 programs in development.

And Moderna has three candidates outside the coronavirus program that are in phase 3 development. If all goes well, two of them -- investigational vaccines for flu and respiratory syncytial virus -- could launch in the coming two to three years. Moderna also recently reported positive data from a trial of its personalized cancer vaccine candidate. The company aims to start a phase 3 trial next year.

Thanks to coronavirus vaccine sales, Moderna has $17 billion in cash to advance these exciting programs.

The case for Axsome

Axsome, as mentioned earlier, has soared this year. Investors clearly are excited about the company's new drug launches and earnings potential.

We got a glimpse of revenue from Sunosi, the sleep disorder drug Axsome acquired from Jazz Pharmaceuticals. In its first full quarter of sales at Axsome, Sunosi brought in $16.8 million. And prescriptions rose 15%. As for the antidepressant Auvelity, it has blockbuster potential. Sales may reach $1.3 billion by 2029, according to GlobalData. Auvelity could carve out a decent share of the market because, unlike most antidepressants, it's fast-acting.

Axsome's pipeline could be a source of even more products not in the distant future but over the next few years. That's because all of the pipeline candidates are in phase 2 or phase 3 development. Two in particular stand out as possible big revenue drivers: a candidate for migraines and a potential treatment for Alzheimer's disease agitation.

Axsome plans on submitting the migraine candidate to regulators in the third quarter of next year. The company recently reported positive data from a phase 3 trial of the Alzheimer's candidate. There currently isn't such a treatment on the market. So this could be a breakthrough product.

Moderna or Axsome?

There's reason to love both of these companies right now. They have launched innovative products and are on their way to commercializing others this decade (if all goes well). A long-term investor may gain greatly by picking up shares of Moderna and Axsome.

If I had to choose only one to buy right now, though, I'd go for Moderna. It's hard to value Moderna by traditional valuation measures until post-pandemic booster revenue starts flowing in.

But it's clear Moderna is undervalued today considering the strength of its pipeline and the potential for recurring coronavirus vaccine sales. Moderna isn't just a coronavirus vaccine company. It's an mRNA specialist that could bring many products to market in the next few years and beyond.

This year's decline in the share price offers us a great opportunity to get in on this biotech company -- and benefit from a new era of revenue growth.