I am the counter to Nightingalesing as I will be attending UNC-CH for Comp Lit next year. Like that poster, I would more than willing to answer any questions you had about the program.
Ultimately, attending an MA program at Dartmouth versus either of the PhD programs is a personal choice, but I would only recommend it if (1) you weren't sure if you wanted to go on for a PhD in general, (2) if you weren't sure if you wanted to go on for a PhD in Comp Lit, or (3) you were facing some pretty dire PhD choices. I am biased, but I don't think number 3 is quite the case.
If however, you feel that your chances at admission at "better programs" (however you choose to define them) would be improved by receiving a master's degree, then by all means go for it. There have been a lot of postings around here about the benefits/negatives of doing a master's first, so I would read up on them before I was too distracted by the Dartmouth name! That being said, I did know someone who did an MA at Dartmouth and is now getting a PhD from Oxford and oddly enough, if you did decide to attend UNC, you would most likely be the only incoming student without a Masters.