Blogs are more discursive, and give the chance to pull info together from different sources. Like my WhittakerLive posting on the Royal Conservatoire of Scotland namechange, or my separate pages on Minstrels and Metaphors.
Tweets are great for single, punchy comments or notifications, and messages to individuals or all followers. And that's why I both blog and tweet, but I have an RSS feed from Twitter to Blogspot, so you can catch up with me both ways!
By the way, you might be interested in Leonard Cassuto's blogpost about academic blogging, in the Guardian Professional's Higher Education Network (advice, insight, and best practice from the community). Does blogging help progress for an early career academic? Read it here.