There is quite a difference between the clouds. The cloud for Green Blog feeds is primarily about environmental issues wheareas the cloud for Green Twitter feeds contains tags for a variety of topics. The second cloud, however, gives insight into the personalities of the tweeters interested in the environment.Seriously, you should check it out.
I touched on this a few months back when I took 5 "social media guru" types and did kinda the same thing. It's also related to what we do at Virtual Vantage Points - taking the discussions of the top bloggers in one online community and examining it over time, or comparing that discussion with that of another online community (like US liberals vs. US conservatives, UK Labour vs. UK Tories vs. UK Liberal Democrats vs. UK Greens, etc.), but VVP focuses squarely on blogs right now (though that may change) and applies subjective analysis from prominent political thinkers.
But Kim's clouds are important because they show how people in the sustainability community use social media tools differently. It's not all the same conversation. I'm taken by the diversity of expression via twitter vs. blogs. Now, maybe if we just took the twitter streams of the bloggers we'd see a more similar cloud, but I don't think so since that's what I did with the social media types. It seems to me that Twitter is used for different purposes and with different results. And it also seems to me that you learn a lot more about the green bloggers by following them on Twitter.
As the marketing/PR crowd starts pitching people via twitter we should keep these fundamental differences in mind.