If you already have a hosting package, Wordpress.org actually works out best financially -- biggest range of free themes and no need to pay extra to use a custom URL or access backend files. In Wordpress.com you need to pay to use your own URL, and pay for premium support if you want to do any backend stuff in a hurry eg get a backup copy of your site files. (It's a two second job on Wordpress.org, took a friend a week to get hers from dot com.)
It's generally preferred to Blogger because of customisation reasons: Blogger users are stuck to a limited layout and theme, without much flexibility for building a site around the blog with contact pages, abouts, book info etc. Whereas Wordpress.com sites have a
large amount of customisation
options which are
really great for
author sites.
It's worth sitting back and thinking about what you want from a site: do you want a simple blog, great google results, no need to pay for hosting? Blogger's the best, being owned by Google, and Tumblr's also a more customisable contender if you're familiar with that system and do a lot of work focused around images (long text on its own doesn't work so well on Tumblr).
Want something a bit more customisable, with more themes and control than Blogger? Want book blurbs, art, any fancy 'proper website' pages? Wordpress.com, or Tumblr with some clever theming to make it more like a traditional blog.
Want to include a shop, heavy theming, or other complex website features (eg a forum, social network) and do you feel confident enough to backup your site and fix your own mistakes? Wordpress.org.
(Side note: I really, really don't recommend Tumblr to anyone unfamiliar with it -- the UI's a disaster. But it *is* customisable and a lot of authors like it, so it felt wrong to leave it out.)