I am often asked this question, which, I have to say, is rather silly, and, my short answer is: Valencia !!!

The long answer is that I much prefer {London AND Paris} to {London} OR {Paris}, while Valencia (Spain) is my new super-cool location with many totally unique advantages - perfect for de-stressing.

All those are very different cities, with very contrasting lifestyles, infrastructures and social operation systems - each is better than the others for certain types of activity.
For business and many other things there is no better place than London, which i consider my hometown.
But - London is stressful and has a few other problems, which can not be solved by other means than occasional escape.

In any case, a regular change of scene (and language) is most refreshing, useful and productive - it re-boots the mind, it refocuses the eye.