1. Blogger.com has simple process of creation of a blog.
2. As Blogger.com belongs Google, blogs on Blogger.com tend to get on top of search Google faster, than blogs on WordPress.com.
3. Options in Blogger.com very easy, practically any can understand with them.
4. Blogger.com allows to earn money, showing AdSense and other advertising.
Minuses Blogger.com:
1. Bad system of comments.
2. You cannot filter a spam.
3. You cannot have some pages if do not know CSS.
4. There are no good standard templates. It is necessary to search for them in a network, or to be familiar with CSS, to make beautiful design.
OR
Pluses WordPress.com
1. You can import pages from blog Blogger.com
2. There is a possibility easily to create additional pages (about a blog, contacts etc.).
3. There are good templates and convenient system of comments.
4. Possibility to subscribe for comments.
5. Preliminary viewing of the comment.
Minuses WordPress.com
1. It is impossible to show advertising, even AdSense.
2. There is no possibility to use plug-ins which has WordPress.org
3. Little bit difficult in use.