Huh, I couldn't tell you. Maybe do both? Take classes you feel are more bread and butter from US sources and one that is an extra, but related to your particular research interest? I know that's easier said than done though; I was rather taken aback by the price of classes. Plus, I have no idea if a UK class would appeal to US graduate schools. I'd think it would be a refreshing example of your cosmopolitan background, but then I'm not on graduate school admissions committees, more's the pity. I feel like it would be a plus in undergrad admissions, but I feel like I have less of a grasp on what grad admissions want than I did on applying to colleges years and years ago.
You know, it's rather rude of the world, really. I was told about what colleges want from the age of being a wee kiddie on the playground, but I'm only now really finding out about grad school requirements (aside from the self-evident, like a BA/BS, good grades, and good GREs).
Did other people have that experience? I feel like my undergrad academic experience was very focused on achievement in the moment rather than do this and that for grad school. Not that I knew exactly what I wanted to do anyway, aside from being sure I would go to grad school for something. Or, did you see undergrad very concretely as a preparation for grad school and planned accordingly as you went along?
Leikcaj, heres the link for the UMass program, so you can take a peep at what they have to offer:
http://www.umassonline.net/