PhD in UK or PhD in Spain?
I don't think anyone is going to be able to give you a straight answer to this question. It will depend on many factors, which only you can really answer:


Do you prefer to live in UK or Spain, and also the city in particular both countries are big and diverse.
The research groups involved and/or the research project (this also includes obviously the supervisor)
The details of the position (salary, possible benefits, etc.)


I probably missed other important factor but the point is this is a personal decision where no one but you will have enough information to answer.