Here's the difference, for me:
I try to use the blog for long, well thought out posts. I don't really care for posts that are simply a link and "Hey, look at this!". Not that I mind linking to other sites, as long as the post does a nice job explaining why it's important that you examine this particular link. I prefer to use blogs as a discussion, an explanation of a topic.
Twitter, on the other hand, has become my fast food post. The 140 character limit allows me to just do a quick "Hey, look at this!" post. Also, people seem to be scanning their Twitter feeds more frequently than their blog readers. Personally, I have almost 1000 unread blog posts in Google Reader. I say almost, because I fight to have it under 1000 as Reader flips a bit and says 1000+. Apparently, I like to know the numbers exactly. But I do scan almost all the 143 people I'm following on Twitter daily.
So, where is all this leading us? Are we truly going down the route, as Feeling Entropy says, of a fast food blogging culture? Now that it is easy, we'll produce less quality? I don't know. But strangely, I feel as if my Twitter feed is having more impact than this blog.
Or maybe that's just because the Twitter effect means I write here less.