The SBL Forum has a post titled: American versus British Ph.D. Programs: Three Doctoral Students Reflect on Their Decisions. I would say they give a good general feel for why one would choose one or the other. I thought the ‘caveats’ at the beginning are also important issues that should also weigh in the decision. For instance the ‘secular’ option was a large draw for me. One other item is financing, which may be a show stopper for coming to the UK. Ultimately, it’s the faculty that is regularly noted as most important. But the ‘holistic’ side noted was important for us, too–opportunities for travel, my kids to gain a broader worldview, etc.
I will say the UK system isn’t suited for everyone. I have a friend here finishing his second year and VERY little written on paper to show for it (he is British though, so he was more likely to be in the UK system anyhow). The review process (at 6, 12, & 18 months) that Durham instituted this year should help cut off some of that procrastination, but it is still ‘independent’ research. On the other side, I’ve gotten plenty of supervision and feel like I’ve moved along decently well. I’ve written about 50,000 words this year–though only 15,000 or so will ultimately go into the thesis.