Short answer: K2.
Why? Several reasons - one, it's free. Two, it does almost everything Superblogger does, and much more besides. Three - it sidesteps the built-in Joomla section > categories > article structure, letting you create instead unlimited nested categories, vs Superblogger, which works within the existing Joomla content structure. Four - it lets you create your own user groups and assign permissions to individual users, making it easy to setup a multi-author blog. Five - really easy to use taggnig system. Six - "K2" just sounds cooler than "Superblogger." Seven - it's free.
That's a quick evaluation. We'll do a more detailed comparison within the next few days.