This issue has been raised in other threads on here (if I recall correctly....a Canadian student was recently implying a Canadian PhD is better than a UK PhD in order to justify his choice to research in Canada rather than here), and the issue tends to relate to the sciences.
I don't know if there really is a tangible quality difference (as distinguished by a difference in the structure of PhD programme)...but I have a feeling a lot it boils down to protectionism issues.
With respect to the Arts, I've never heard anyone mention a quality difference and I do know a lot of British PhDs who now hold assistant. prof. posts in America.
However, if you plan to pursue an academic career in the US, it's probably more practical to have a US PhD.