According to the Pew Internet & Amercan Life Project, e-reader ownership outpaces tablets:
"The share of adults in the United States who own an e-book reader doubled to 12% in May, 2011 from 6% in November 2010. E-readers, such as a Kindle or Nook, are portable devices designed to allow readers to download and read books and periodicals. This is the first time since the Pew Internet Project began measuring e-reader use in April 2009 that ownership of this device has reached double digits among U.S. adults.
Tablet computers—portable devices similar to e-readers but designed for more interactive web functions—have not seen the same level of growth in recent months."