Well, are you going to be receiving any funding from Tufts? And are you planning on applying for a PhD afterward? I was in the exact same situation a few weeks ago, trying to decide between getting my MA in Classics at UCL or at a university stateside. I would have had funding here, but my boyfriend is English, and so that was a whole separate issue to consider. Luckily for me, I have just recently been accepted into a PhD program here, with funding, so my decision has become quite easy to make. But when I thought I did have to make the choice, I spoke to several of my advisors about it, and received (of course) all sorts of different advice. A couple of professors felt that it might be quite a good option for me to attend UCL, but that I would have to reapply to MA/Phd programs here for the next year - so basically I would end up with two MA's. Another professor thought that I would be able to enter a PhD program straight from my UCL MA, but that didn't seem to be the general consensus.
Will you have any teaching opportunities at Tufts? You definitely won't at UCL, so if you will at Tufts that's something to consider.
Additionally, you also should consider the cost of living. While paying for a UK MA might be cheaper, living in London is not. But it is an amazing city, and you will have many many resources available to you academically. Honestly, the real question here is funding. If you're going to receive any sort of funding at Tufts (even just tuition remission), I would go there. The only reason I struggled with my decision is because my boyfriend's entire network of family and friends lives in the UK, and he's been sticking it out here with me for the last five years.
One last little thing to think about - if you ARE planning on applying for PhD programs, if you attend UCL you will have to apply in the fall. After going through applications this year, the idea of having to do it all again in a few months was almost sickening. Going to Tufts at least grants you a years break from application hell (unless you're not planning on applying straightaway, but are going to do fieldwork or something for a couple years).
Anyways, while lengthy, I'm sure my reply is not especially helpful, since I was just as stuck as you are. But good luck with your decision, I'm interested to see what you do!