Sefa Tsegha ∙ 42 weeks ago
If you intend moving your blog to WordPress, then the earlier the better. You can save yourself traffic lost when you move after your blog had already gained a lot of it.
WordPress has many benefits over blogger. I would advice you self-host your WordPress to fully take control of your blog. Nevertheless, that depends on your budget.
Site ranking has many factors, so one cannot conclude just because your site is hosted under Google's domain, then it will rank high. Moreover, WordPress has lots of plugins to help your blog rank high, especially if you have good content.
I have used Joomla, Drupal, Blogger and WordPress. However, I will always recommend WordPress over the others.