Mathematical creativity: better a high profile or a "laid back" department? It seems that high profile universities are not the most enjoyable work environment to be in.
Even though big institutions attract big names and grant money, it seems to me that the pressure to perform leaves everybody, from the undergraduates to the professors, overworked.  On the other hand, the idea of less prestigious university is attracting me for the, perhaps overly romantic idea, that as passionate people could be working in these institutions. 

I am thinking: perhaps moving out of a big institution to a smaller one could increase quality of academic life, as well as creativity. But surely quality of science need not be inversely proportional to department enjoyability?

-Is this a realistic expectation?

-Does this depend on the research interest?

-Does achieving a PhD in an impressive institution open many doors that would otherwise remain close (for example, being able to be an independent researcher)?

Additonal info:

I have graduated from a MSci in Theoretical Physics in a big London university, and I am currently doing a MSc in Applied Mathematics there. Interests were fairly broad and decided to prolong my taught education to "taste" more subjects.
   I love Physics and Maths almost as much as air, but I am a bit concerned with the dullness of my current environment. I haven't been to other universities, or know many passionate people from other universities, so I have not had the chance to ask these questions to many people.
Let me focus on the question in the title.  In my experience, a vibrant mathematical community is the best environment for creativity.  More prestigious universities/departments tend to attract creative and passionate people, who create vibrant communities, and being surrounded by them will help you be more creative and passionate.  While this is not a rule, and some departments just have bad atmospheres (you need some individual knowledge of the department to know about this, so visits etc are good), you're more likely to find inspiration at a place with cutting-edge research, than at a place where most of the research is "pedestrian."

There is a caveat, however.  If you can barely keep you head above water, it's hard to be creative.  Fortunately, most places won't admit you if they don't think you can handle it, though it does happen.  Again a visit, or browsing student/faculty webpages/blogs may help with this. See also this related question on MSE.