Never in the history of book marketing has there been a crusade quite like Sheryl Sandberg’s. Last Thursday in New York, the Facebook COO hobnobbed with former U.S. Treasury Secretary Tim Geithner, film director George Lucas and Barbara Walters at a party in her honor hosted by Mayor Mike Bloomberg and Arianna Huffington. Friday morning at JP Morgan Chase headquarters, she dined between CEO Jamie Dimon and her parents, and then evangelized about empowerment to executive women. Sandberg and her new book, Lean In: Women, Work and the Will to Lead, were featured on Sunday’s 60 Minutes on CBS. This week’s Time features a thoroughly self-possessed Sandberg standing behind the cover line “Don’t Hate Her Because She’s Successful.”

Meanwhile, 3,000 miles away in Silicon Valley, Yahoo CEO Marissa Mayer hunkers down, refusing to talk publicly about her controversial no-work-from-home policy and doing all she can to stay under the radar.

As much as Sandberg strives to be a role model for the new generation of aspiring women, that role is precisely what Mayer does not want to be.

Pardon me for comparing the two. It’s irresistible.

First, let’s acknowledge their similarities. Besides being Silicon Valley’s most powerful women, Sandberg and Mayer are the two most famous executives, male or female, to leave Google . They’re the fastest-rising stars on Fortune’s annual Most Powerful Women list (Sandberg is No. 8 in our 2012 rankings; Mayer is No. 14). And thanks to Sandberg’s Lean In crusade (parties! speeches! TV appearances! Lean In circles!) and Mayer’s new HR policy, these two women are challenging the norms of how we behave in the workplace.

There ends the similarities. To understand how different these two women are, you have to know where they came from. Sandberg is the daughter of a Florida ophthalmologist and the sister of two doctors . Her dad, Dr. Joel Sandberg, told me at the JP Morgan Chase breakfast that Sheryl is the odd duck in the family—graduating from Harvard Business School, charting a career in government (working for Larry Summers at the World Bank and then at the U.S. Treasury), and scoring big in business. It was forceful persuasion by Eric Schmidt (the best advice she ever got, she says) that pulled Sandberg off the government track. “Go for growth,” then-Google CEO Schmidt said to woo her to Silicon Valley in 2001. “Growth moves everyone up. If it’s growing it works.”

Sandberg, 43, and Mayer, 37, were friendly but not close to one another at Google, in part because they worked on different sides of the company. Sandberg was all about money and scale—building the global sales and operations teams that has helped make Google a financial machine. Mayer, an engineer’s daughter who has a Masters in computer science from Stanford, joined Google straight out of grad school in 1999. The company’s first female engineer, she worked 100-hour weeks directing the look and feel of the products, enjoying pretty much every minute of it.

Both women dreamed big. But while Sandberg quit Google in 2008 to become Mark Zuckerberg’s No. 2 at Facebook, Mayer stayed, trusting that great work would propel her inside the company. Admittedly a shy geek, Mayer told people that she views a career as “a step function: When you’re ready to take the next step or take on more responsibility, you should start doing your job at the next level. The promotion will come naturally.” Her philosophy may have been naïve, since Mayer got sidelined in her last couple of years at Google. Last summer, when Yahoo’s board was looking for a CEO, she was pregnant and ready to step into a big new job.

Sandberg, who famously leaves the office at 5:30 pm most days to go home to her two young children, is a proud feminist. She adores Gloria Steinem, leans on her CEO husband Dave Goldberg to share housework, and considers Lean In “a sort of feminist manifesto.” Mayer, who is only seven years younger than Sandberg but sometimes seems to be of another generation, says, “I don’t think I would consider myself a feminist.” In
Makers
, the PBS documentary about the rise of women in America, she explains that she considers “feminism” a “negative word.”

And while Sandberg endeavors to change the world, Mayer has other priorities. She is singly focused on turning around Yahoo. Well, maybe not singly focused. Her priorities, as she told me last fall: “For me, it’s God, family and Yahoo—in that order.”

Mayer has declined to speak publicly about her new HR policy—and last week I criticized the delivery of her message announcing the no-work-from-home decision. A couple of things worth noting: The policy affects about 1% of Yahoo’s workforce. Mayer is allowing exceptions. (She and EVP Jackie Reses, who oversees HR, have been evaluating requests this past week.) Managers who want to keep high-performing telecommuters will, in certain cases, permit some work from home or they will pay to relocate employees to a Yahoo office. As Best Buy disclosed last week that it is revamping its policies along the lines of Yahoo, the widespread anger over Mayer’s decision began to dissipate.

As for the Yahoo CEO’s “nursery” for her son, Macallister, I’ve seen it. It’s a room, as plain and basic as can be, next to her corner office in Sunnyvale, Calif. Mayer refurnished her office, as well as the nursery, out of her own pocket; she bought shelves at Ikea and a round table for $200 on Amazon.com.

When the Yahoo directors hired Mayer, six months pregnant, last July, they told her, essentially: “Do what you need to do to turn the company around.” She’s working on it, her way, and so far has lifted the stock price 45%.

It’s fair game to critique her policies and even her behavior as Yahoo’s chief executive, but let’s acknowledge that she, though a reluctant role model, gives us another way to envision female bosses succeeding. In other words, there are different ways to lean in.