I know someone who went through the cam mphil, and the difference between Cambridge and tufts seems clear to me. Cam is a very short, one year program, which means there's little time for you to fully explore your interests, and you only complete three essays (cannot recall well) during it. People there seem to go straight into their research topic almost from the beginning. So not bad if you already have something in mind and really want to work it out with some professors there.
Tufts is a two year program, where you do 12 graduate-level courses (woww), and probably also some TA work. Just think of it as the first two years of a US PhD program. I do understand if you don't want to take a wide range of things again, but I suspect no M&E will do little benefit to your later PhD applications to US schools. This is less an issue for UK PhDs though.