PhD in UK or PhD in Spain? I have won fully funded Ph.D. positions one in the UK and the other one is in Spain. 
UK job position offers me a job as a teaching assistant in addition to Ph.D., however, I would have a fellowship (research assistant) in Spain? 

Ph.D. is itself is a big commitment and requires focus and hard work. I want to choose something with which I can focus on my studies as much as I can. Yet, on the other hand, I want to have professional experience in academia for further career development. 

I have to choose one now and I am not sure which one would be better in the long run? 

Suggestions would be appreciated!
Thank you!
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.