I don't know about the high vulnerability of Sununu here are the Congressional Quarterly rankings. http://www.cqpolitics.com/risk_rating_senate.html. I still rank Coleman ahead of Sununu as vulnerable, unless Shaheen jumps in. We will have to wait and see a poll out of NH concerning the race.
I think New Hampshire just edges Minnesota on two counts. First, because Coleman seems to be more popular than Sununu and also because the quality of candidates in New Hampshire seems better than in Minnesota - I just think Al Franken is too much of a loose cannon to run. Plus, 46% of Minnesotans already have a negative impression of him. I do not know much about Mike Ciresi - but he does not strike me as a great candidate.
There is also the fact that New Hampshire appears to be swinging Democratic recently and the overwhelmingly popular Democratic Governor John Lynch is on the ballot again in 2008.