I've had this blog now for some years. I also recently put up my professional site on google sites. Very recently I've started to set up a blog/site on Wordpress.com
One thing I've noticed for both blogger and sites is that the pages are just not that pretty. I dislike fancy gadgets and tend towards minimal themes. Compared to blogger/sites Wordpress somehow gets the typography and layout just right. Wordpress sites are prettier, for reasons I can not explain.
This is a bit of a pity. Blogger lets you customize EVERYTHING. This is awesome. But no matter how I (an complete amateur in design) tweak, I can not get a pretty enough blog. Sites is horrendous. Everything looks very dull and drab.
Wordpress charges for some themes, and will charge you a bunch to have access to customizing the CSS and layout. It's not as great as free, but in a way I think the fact that Wordpress charges is a good sign. It means that this is a source of income for them. Which, in turn, means that they have an incentive to make things pretty and easy to use and cater to a wide variety of visual tastes.
In terms of usability, Blogger is really easy to use, though the interface can be a bit clunky. Wordpress is as easy to blog on and the interface is pretty.
Google sites, though free, not only looks bad, is very clunky to setup. Wordpress allows static pages as part of a blog (which you can easily turn into the main feature of your site) and this allows you to make very pretty sites.
This whole experience with blogger and wordpress has been interesting and informative. Blogger is a free service given away by google. It is technically very good, but visually not so polished and the interface is passable. Google sites is technically good, but visually unappealing and its usability is horrendous.
Wordpress (the site) offers a basic free version that is good enough visually. It makes money off sites that are willing to pay for more control over appearance. I think since wordpress generates revenue through selling the service/product itself they have paid more attention to visual polish and usability.