I guess it comes down to the differences between a British PhD and an American PhD (which have probably been discussed in more detail on another thread).
But, in a nutshell, in an American PhD, you get to take classes for at least a couple of years before starting dissertation research. In comparison, in a British PhD you jump right into the dissertation. So, you should think about where you are academically, and what your needs are: do you feel like you need course work, or are you ready to start your dissertation work?
Then, there is the job market to think about. Do you see yourself working in the US or in Britain/Europe? Which program is right for you will depend on the answer to that question.
Another issue to consider is teaching. Generally, in an American program you can get TA experience (though not always), whereas in British programs it is rarer to come by. So, you should think about whether teaching experience is something you are keen on developing or not.
Finally, apart from all of these differences, a big part of your decision will come down to your individual field, adviser, and department. You didn't mention what your field is, but maybe one of these departments/advisers is stronger than the other for that particular area?
