I have an opportunity to do a PhD in a top-tier university with a brilliant professor who I know also is a very good mentor. This is in spite of the fact my early undergraduate years were less than stellar, so my cumulative GPA is not that competitive.

This all is quite amazing, however, in being completely honest with myself, the subject matter and methods of this particular lab are not exactly where I am strongest or even the most excited about. I am not completely disinterested - it is research like I have been doing for the past few years and enjoyed, but I don't think it is what I would want to spend my life doing. 

What is your opinion on taking this offer versus pursuing a topic I feel is more suited to me? If I take the offer, I imagine I would be simultaneously trying to advance myself in the types of methods I am most interested in on the side and would not be getting a bunch of help from him in these particular areas. Then after I graduate maybe I could switch my focus? 
Or alternatively, I don't take it... I'd have some catching up to do to switch fields now and would probably end up in a much more mid-tier university. 

I am in the social sciences, in case that matters.