I like both Blogger and WordPress, and they're both great, free services. But I think I like WordPress a bit more for how I work.
Specifically, I'm moving for the following reasons:
- Better previews of posts. This is important when attempting certain layouts in a post.
- Better support for pictures. I like the direct integration in WordPress, rather than Blogger's bridge into Picasa.
- Threaded comments
- Active and regular development
It also seems like WordPress is more about selling upgrades, so site customization is an extra fee. That's a bummer, as I like creating my own layout and CSS style sheets.
So, nothing's perfect, but free blogs are great, so just choose one and enjoy.