I would have to say wordpress. Many of the Top Bloggers currently, all use WP and they all have at least 20k posts. Most of them have been blogging for years and were the original bloggers online. I personally use WP too and love it.
i have my blog in blogger.com, and face some limitation 1 - when i search any previous post, the blog search only with in new 5k post, they did not search my old posts, so there is limitation in search posts. how can people search my old post? 2 - we did not sort posts under particular label in alphabetical order. 3 - label limit is 5000 4 - we only export 5k posts in blogger. Self hosted wordpress blog also face these bugs
I would go with Wordpress every time. Seems to have the most plugins, themes etc available which is always a bonus. Ive done sites that big in WP and never had a problem
Server load wise, wordpress is best. Others out there allow better database systems, but overall MySQL is the best.
I personally use both blogger and wordpress. However, I have found wordpress much easier and it has also some great features.
wordpress vs blogger: i will choose wordpress. joomla vs wordpress: joomla is CMS more than blogging platform so go for wordpress if u r looking for blogging solution. Pro bloggers techcrunch, smashingmagazine, problogger and other big names are using wordpress.