I would opt for the UK because i'm from europe and don't plan on going or moving to the US, so i think UK would be a more logical choice. I would expect the PhD process to be equally intensive and hard work, and the end result to be of equal validity and value. that being said, if for example you have a big name university wanting to take on your phd project in either and a smaller name in the other, you might opt to go for the big name because it will look more fancy on your resume if a person looks at it who is not in-depth in the field and doesn't know how to compare them apart form knowing some big names.
For you i would suggest taking a look at your future and seeing where and what you want to do there. if you want to do something in the US i would recommend following in the US as employers there will probarbly be favorable to US based universities as obviously some parts will be more tuned to the US market and academic culture, where the UK will be more tuned to their own environment there.
I think in the end your own personal input is what makes a phd valuable and good, as it's your own research project and it's in your hands what quality you will deliver. it will be judged on correctness and all sorts of things, but you are the master of your own project and research so it's your own input which matters most. this will be the same in the UK and the US.
I would go for one based on my future plans and geographic location of those plans more than 'quality' of the establishment, as it's not a course you will be following, but your very own hard work which will be the value and object of the Phd.