Editor's note: Amy Gahran writes about mobile tech for CNN.com. She is a San Francisco-area writer and media consultant whose blog, Contentious.com, explores how people communicate in the online age.

(CNN) -- Ads touting Apple's iPad seem to be everywhere, but e-readers such as Amazon.com's Kindle and Barnes & Noble's Nook are actually more popular with consumers, according to a new report from the Pew Internet and American Life Project.

Last winter, tablets had a slight market lead. According to Pew, as of that time, 7% of U.S. adults owned a tablet computer (such as the iPad or Motorola Mobility's Xoom), while only 6% owned an e-reader device.

But that picture soon changed drastically. By May, 12% of U.S. adults owned an e-reader, while tablet ownership expanded only to 8%. (Note: the margin of error on this survey is 2%, but that would not challenge the market lead of e-readers.)

This is not an either-or technology choice. Pew noted that 3% of adults own both devices. Specifically, 9% own an e-reader but not a tablet, and 5% own a tablet but not an e-reader.

Apple has sold more than 25 million iPads and has a dominant share of the tablet market. Amazon and Barnes & Noble don't disclose sales of their e-reader devices. Citi analyst Mark Mahaney forecasts that Amazon could sell 17.5 million Kindles in this year alone.

Who's buying e-readers? According to Pew, Hispanics (who appear to be leading other U.S. ethnic demographics generally in embracing mobile technology), adults under age 65, college graduates, parents, and people in households earning less than $75,000 per year are especially likely to own e-readers.

Also: "There was considerable growth in e-reader ownership between November 2010 and May 2011 among college graduates, one-fifth of whom now own these devices," Pew reports. This is interesting, since Insider Higher Ed, an industry trade publication, recently reported that after a slow start, the market for electronic college textbooks experienced a surge this spring.

Who's buying tablets? Pew reports that from November 2010 to May, the largest increases in tablet ownership have been among men, Hispanics, people with at least some college education and household incomes of $30,000 or more. But the very highest increases in tablet ownership were seen among Hispanic adults and households earning at least $75,000 annually.

Why are e-readers more popular than tablets?

Based on device specifications alone, tablets would seem to offer greater consumer appeal, since you can do far more with a tablet than an e-reader. A tablet is an e-reader plus a video screen plus a game player plus a web browser plus ... .

E-readers typically limit users to buying, downloading, reading and annotating books. Furthermore, most e-readers still feature a black-and-white e-ink display and lack touchscreens. (Though, the new Nook has the latter.)

But price may play a key role. Right now you can buy a brand new Amazon Kindle for as little as $114, with free Wi-Fi access -- and Amazon has reportedly hinted that some day the Kindle might be given away for free. You can get the simplest Barnes & Noble Nook for $139. (Can't decide? Consumer Reports gave the Nook slightly better marks than the Kindle.)

In contrast, the least expensive iPad 2 costs $499. If you want 3G data access, the entry-level model costs $629. (New or used first-generation iPads are sold for less.) Adding to the total price, two wireless carriers offer prepaid or month-to-month data services: Verizon's iPad data plans start at $20 a month; and AT&T's plans start at $15 a month.

The BlackBerry Playbook costs $499. Many Android tablets also sell for that price, although the Asus Eee Pad starts at $400.

There's a fuzzy line between e-readers and tablets. Most notably, the $249 Nook Color, which features a touchscreen, advanced Web browser and Wi-Fi chip, is actually a modified Android tablet. The blog Tech Republic explains how to hack the Nook Color to operate as a full-feature Android tablet.

Of course, second-hand, refurbished or older-model e-readers and tablets of all types are widely sold via eBay, Craigslist, and discount vendors for considerably lower prices.

E-readers and tablets are still very much emerging markets.

"Both e-book reader and tablet computer adoption levels among U.S. adults are still well below that of other tech devices that have been on the market longer," according to the Pew report. "Cell phones are far and away the most popular digital device among U.S. adults today, followed by desktop and laptop computers, DVRs, and MP3 players."

For some additional perspective on mobile technology, Pew writes: "This survey marks the first time that laptop computers are as popular as desktop computers."

The opinions expressed in this commentary are solely those of Amy Gahran.