China’s e-commerce leader Alibaba Group plans to go public next year, likely on a U.S. stock exchange, and analysts estimate the company may be valued at around $100 billion, close to the market valuation of Facebook.

And while this valuation is a little more than half that of U.S. e-commerce leader Amazon.com, which has a market cap of $170 billion as of today, the Chinese online retail giant booked nearly double Amazon’s sales in 2012, according to Internet Retailer estimates. Further evidence of Alibaba’s dominance in China came on Nov. 11 when the company brought in $5.75 billion in sales in a 24-hour period during its annual “Singles Day” sales event, which is the Chinese version of Cyber Monday in the U.S. This is more than three times the $1.46 billion all U.S. consumers spent online on Cyber Monday 2012, according to comScore Inc.

The leaders of the two largest e-commerce markets in the world—China and the U.S.—already compete directly with each other in China, with Alibaba’s Taobao and Tmall marketplaces going head to head with Amazon.cn. As Alibaba expands, aided by the billions it is likely to raise in its upcoming IPO, the two e-commerce giants no doubt will also compete in other markets, including the U.S.

With this in mind, Internet Retailer draws on company reports and its Top500Guide.com data to break down the similarities and differences between the two online retail giants.

One basic difference between the two is that Alibaba only provides an online shopping marketplace where other retailers sell, and does not sell directly to customers. In contrast, Amazon both sells merchandise that it owns to customers and also operates a marketplace. The key metric to look at here to compare the marketplaces is gross merchandise value, or the dollar value of all items sold on each platform. Alibaba reported that the volume of transactions on its two marketplaces, Taobao and Tmall, was around $170 billion in 2012.

Determining Amazon’s GMV, however, requires some guesswork, because Amazon does not report it publicly. Instead, it reports total revenue, which was $61.09 billion in 2012. Some of that came from services unrelated to consumer purchases, mainly Amazon Web Services, a business unit that rents data storage capacity and computing power to other companies. Amazon doesn’t disclose AWS revenue, but analysts estimate this at about $2 billion for 2012.

That would leave about $59 billion in revenue from sales of Amazon-owned merchandise, and the cut it takes from third-party sellers on its marketplace. Amazon has disclosed publicly that other retailers selling on its marketplace account for about 40% of the units sold on Amazon sites. And Scot Wingo, CEO of ChannelAdvisor Corp., a marketing company that helps retailers sell on Amazon and other web marketplaces, estimates Amazon takes an average commission of 10% on sales by third-party sellers. Doing the math, that suggests Amazon’s own sales totaled just over $55 billion and sales by marketplace sellers $37 billion (with Amazon taking a 10% cut and booking some $3.7 billion in revenue). That would put Amazon’s 2012 GMV in the range of $92 billion, or just over half of all sales on Alibaba.

Part of the reason that Alibaba is transacting nearly double the amount online than Amazon is that the Chinese company is serving a much larger pool of online shoppers. As of the end of 2012, the U.S. had 254.3 million Internet users, and China more than double that at 571.1 million, according to the World Bank. And China has more room to increase its base of web shoppers, as only 42.3% of its population uses the Internet, compared with 81% of U.S. consumers.

In addition, Chinese people use the web differently than U.S. consumers do. “Compared with U.S. people, Chinese people prefer engaging with others through social media and mobile,” says Sun Baohong, a professor at the New York City campus of Beijing-based Cheung Kong Graduate School of Business.

The numbers bear out that assessment. As of the end of June of 2013, there were 464 million mobile Internet users in China, according to the China Internet Network Information Center, three times the number in the U.S. In addition, the most popular Chinese social platform, Qzone, owned by Chinese internet giant Tencent Holdings Limited, had more than 600 million subscribers in the middle of 2013, Tencent says. In contrast, Facebook reported around 200 million U.S. subscribers at same time.

In addition to total sales volume, Amazon and Alibaba tend to diverge on such key operating metrics as average order value. The average ticket is $178 on Alibaba, while Amazon’s is a little higher at $220, according to data available on Top500Guide.com. In the past year, Alibaba has expanded into many new categories, such as automobile and financial products, so its ticket price may increase in the future. Amazon has also been moving into more luxury categories, and its average ticket may increase as well.

Amazon’s top category is books and other media, such as music and videos. In the third quarter of 2013, media represented 30% of sales on Amazon’s sites, Amazon reported. By contrast, the dominant category on Alibaba marketplaces is apparel and electronics. In the recent Nov. 11 sales event in China, four of the top 10 sellers on Alibaba were apparel brands, and three others sold electronics.

In one area, however, these two giant e-commerce companies are moving on parallel tracks, and that is in mobile commerce. Both recorded a similar percentage of sales from mobile devices in 2012—6.5% for Amazon (or $4 billion out of its $61 billion total, according to Top500Guide.com) and 7% (or $12 billion out of $170 billion) for Alibaba—and the percentages are moving up for both. In the recent Single day sales event, 15% of Alibaba’s sales came from mobile devices. Amazon’s mobile sales will increase 100% to $8 billion in 2013, Internet Retailer estimates.

Updated 2013 e-commerce sales figures and other key metrics for Alibaba and the 499 other largest online merchants in China will be available on Top500Guide.com in mid-February.