Warren Buffett is the greatest investor of our time, so when he invests in a particular stock, it pays to find out why. Buffett is famous for his focus on value stocks -- companies trading at relatively low valuations compared to their earnings and long-term growth potential. But he and his team are also increasingly choosing growth companies, businesses that are taking big steps forward in their sector -- often with the price appreciation to match.

Here are two Buffett stocks that are worth adding to your portfolio. The first is Costco Wholesale (COST 0.07%), a consumer defensive retailer with a business model that is perfect for these uncertain times. And the second is StoneCo (STNE -0.15%), a Brazil-based fintech with plenty of room to expand in its market. Both stocks would make great buys right now.

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1. Costco Wholesale: Consumer defensive cash flow

Berkshire Hathaway bought 24 million shares of Costco back in 2000. After trimming its initial position, Berkshire now owns 4.3 million shares, giving it a roughly 3% stake in the warehouse retailer. At Monday's share prices around $340, Costco's stock has soared almost 1,100% from Buffet's entry point of $28.62 per share. And the company looks poised for continued growth because of its consumer defensive business model and customer retention.

Costco passes on savings to customers by eliminating overhead and keeping operations as streamlined as possible. While consumers must pay an annual membership fee to shop at Costco warehouses, they can enjoy massive savings if they buy items in bulk. The strategy is working. In the fiscal third quarter, sales jumped 7% to $36.451 billion while the membership renewal rate stood at 91% in the U.S. and Canada.

Costco's membership renewal rate is a key metric to watch because once consumers pay for a membership, they are incentivized to maximize their shopping at Costco -- boosting same-store sales.

Costco generated a net income of $906 million in the third quarter, which is 8% lower than the prior-year period due to the coronavirus pandemic's negative effect on margins and the company's travel booking business. However, this crisis has created an opportunity for Costco to drive e-commerce adoption, which soared 64.5% in the quarter due to stay-at-home demand for goods and services.

2. StoneCo: An emerging market growth opportunity 

StoneCo provides financial technology solutions to online retailers in Brazil -- an e-commerce market projected to expand at a compound annual rate of 7.5% until in 2024. Berkshire bought shares in StoneCo when it went public in October 2019, and the stock has roughly doubled since then (as of Monday).

StoneCo is poised for continued success because of its massive market opportunity and innovative business model.

According to McKinsey, 47% of payment transactions in Brazil are carried out in cash, and up to 55 million adults are still unbanked -- leaving ample room for StoneCo to compete for market share as the country's financial system modernizes. Analysts at Goldman Sachs believe Brazilian fintechs can also take business from traditional Brazilian banks due to their technological edge and improved efficiency.

To compete in a country where face-to-face interactions reign supreme, StoneCo has invested in a massive brick-and-mortar footprint that brings it closer to its clients. These physical locations are called Hubs, and they offer service, sales, and operations teams that can tailor solutions to local clients' needs.

StoneCo is also expanding within the fintech vertical through specialized software like Mlabs, a social media e-commerce platform, and Delivery Much, a food delivery marketplace. In August, the company completed a business combination with Linx, an integrated business management software company focused on retail clients. The combination looks highly synergistic and should help to boost long-term growth.