Commission-free stock trading platform Robinhood has become a resource for investors looking for their next big bet, and its list of top 100 stocks holdings among traders using its app is a valuable heatmap of where retail investors are putting their money.

Not every top-ranked stock on Robinhood's list would be considered a good investment among veteran investment advisors, but two of them -- Peloton Interactive (NASDAQ:PTON) and Zynga (NASDAQ:ZNGA) -- certainly qualify.

Let's dig a little deeper and see if we can find out why these growth stocks make Robinhood's top stocks list, and why the two could make great additions to your portfolio in October. 

three green arrows hit the center of a target that has a large dollar bill sign on it.

Image source: Getty Images.

1. Peloton Interactive: A highly scalable business model

With a market cap of $32 billion, annual revenue of only $1.83 billion, and a net loss of $71.6 million, Peloton Interactive's financials suggest it is not a cheap stock. But despite its lofty valuation, the fitness company could still be a compelling investment because of the consumer trend toward at-home fitness. To add icing to the cake, Peloton's subscription-driven business model can deliver significant margin improvements as it grows in scale. 

The coronavirus pandemic may have permanently changed consumer behavior in favor of at-home experiences. According to a survey conducted by Wakefield Research, some 90% of Americans plan to continue at-home workouts even after they feel comfortable returning to traditional gyms. Peloton is already reaping the benefits of this trend, which is boosting demand for the company's workout equipment and subscriptions. 

A man exercises on a Peloton stationary bike in a home while a woman walks by him into another room.

Image source: Peloton.

Fiscal 2020 revenue grew 99.6%, with connected fitness products (hardware) representing around 80% of the total. Subscription sales made up the other 20%, and will be a key driver of margin growth over the long term. That's because Peloton's cost of producing bikes and treadmills has tended to increase directly with the number of units sold, while subscription margins improve every year. 

Peloton reported a gross margin of 43% in its connected fitness segment, compared to a 57.2% margin in the subscription segment as of 2020. The subscription segment's gross margin has grown from 43.3% since 2018, while the connected fitness margin has remained comparatively flat over the same period.  

Zynga: An acquisition-driven growth strategy 

Video gaming has evolved from a niche hobby into a multibillion-dollar industry, and the coronavirus pandemic has helped bring it further into the mainstream. Video game sales are surging, with a 37% year-over-year gain in August, and Zynga looks like a compelling way for investors to bet on this trend. The company's acquisition-driven strategy could help drive its continued success.

In 2018, Zynga purchased ownership stakes in mobile game developers Gram Games and Small Giant Games for $250 million and $560 million, respectively. These acquisitions gave the combined company access to Empires & Puzzles and Merge Dragons!, two assets that together represented 43% of Zynga's online game revenue in the second quarter and helped power quarterly growth.

Zynga released the "Harry Potter Puzzles & Spells" game in 2020.

Zynga launched the "Harry Potter: Puzzles & Spells" mobile game in September 2020. Image source: Zynga.

Total revenue grew 47% in Zynga's most recent quarter, as strength in the online game segment offset weakness in the advertising segment, which declined 3.5% to $63.5 million in the period. 

Management plans to boost its advertising business by investing in hyper-casual games -- super simplistic mobile apps that are monetized by ads instead of in-game purchases. In October, the company's closed its acquisition of Rollic Games, a Turkish hyper-casual game maker, for $180 million. According to Zynga's CEO Frank Gibeau, eight of Rollic's games have reached number 1 or 2 in the U.S. app store. He expects the deal to boost Zynga's advertising business going forward. 

Zynga reported a net loss of $150.3 million in the second quarter, as the new investments led to expanding operating expenses. Losses are not uncommon for growth companies, but Zynga will need to keep expenses in check to unlock the maximum value from its acquisitions. 

Which company is better for your portfolio?

Is it Peloton, with its rich valuation and disruptive business model, or Zynga with its acquisition-driven growth strategy? Both stocks could make slam-dunk investments in October and beyond, because they benefit from consumer megatrends toward digital experiences and have convincing strategies for continued expansion in their markets. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.