Hiya. I began my PhD in the US and transferred to a UK university after I moved here.
As I too hope to return to the US someday, before I made the switch I asked several colleagues in the US their opinions about UK PhDs. No one seemed to have any preconceived notions. I think that the name of the university you attend, the reputation of your supervisor, and most importantly the quality and topic of your own research really decide how respected your degree will be in either country.
Regarding differences between programs in the two countries, I think there are a number of pros and cons. As someone else has already posted, the most influential factor in your PhD experience will be your supervisor (aka "advisor" in the US). I actually had a British supervisor when I was enrolled in the UK, and I transferred to an American supervisor here! My impression is that British supervisors and the British system in general, tend to be more hierarchical than in the US, where people are less formal and less class conscious and grad students and professors mingled at social activities as well as department events. I think attending classes in the US also encouraged more contact and comradery among grad students, which I really miss here. That being said, I expect to finish my PhD in less than half the time that it would have taken me in the US, so perhaps the solitude is worth it. Similarly, I think that taking classes was worthwhile, but I don't know if the time trades off. I think that being a TA in the US was very worthwhile-- that's probably the most important thing that I find missing in UK PhDs: teaching experience-- but again, this is one of the things that makes US PhDs take so long.
So to sum up my ramblings, I think that the PhD "experience" in the US is fuller and generally superior, but I find the speed of the UK PhD very compelling as well. And I can't emphasize enough how important your supervisor is in either country.
Good luck!
