I think that Tufts isn't ranked because USNews only ranks official "schools" of public health. Tufts is CEPH accredited and has both an MPH and a DrPH program.
The advantages of Tufts is that there are small class sizes, I think with a limit on how many students can enroll in a class, and you might well get to know your professors better at Tufts. Tufts is in Boston, the same as BU, so going to one versus the other probably has no bearing on getting a job in Boston which is an easier place to get a public health type job. As some posters on this forum, who hire MPH grads, have said, they might not even look at where you got the MPH, so I think factors such as having small classes and friendlier faculty at Tufts, versus possibly getting lost in the crowd at BU, should be considered. People interviewing at BU have noted that the faculty don't seem really interested in the students, and BU's curriculum was recently changed to mimic that of Columbia anyway.