I don't think tweeting and blogging can be compared really. I would see tweeting as a form of "talking to a group of people". You often don't research, proofread, and rewrite yourself when talking. The UI to tweeting also boosts this (mostly mobile devices I assume, via a small text box).

While bloggin is for writing an essay. You may write the essay and just publish it, but in most cases you do some research and at least proofread it once. And again the blogging UI is optimized for this: you have an empty page, nothing other than your written content.

And they really complement each other: you talk to people to get ideas for your essays, and you write essays to share your ideas with people and use them as the base for your writing. I don't think you ever can replace tweeter (or similar services) with blogging.