Because Darwin's "Origin" may be published by anyone, there are various editions available that seemingly differ only in price and introduction. In fact, however, among the various published versions of the "Origin," there is a difference vastly more important than price and intro -- that is, which *edition* is being published. Harvard and Penguin publish the first edition of the "Origin," whereas Bantam, Modern Library, and Prometheus Books publish the sixth edition.
For almost every purpose, the first edition is the only version worth reading. Aside from its overwhelmingly superior historical merit, the argument in the first edition is shorter, livelier, and more persuasive than the one in the sixth edition, where Darwin includes concessions to physicists such as Kelvin, which were ultimately proven unnecessary (as Kelvin's claims were shown to be in serious error). These erroneous concessions forced Darwin to mistakenly add several non-Darwinian arguments to his later editions.
So, if you want to read the book that changed biology forever, then read the "Origin" as it was originally written: buy the Harvard or Penguin copies.