Postdoc without restrictions
Congratulations! It sounds like you have two very reasonable options. A few questions for you to consider:

First, to evaluate the risk of getting lost, ask: how independent do you think you are capable of being right now? In your  graduate school projects, think about how much input you had both in developing all aspects of an idea and carrying it out. The ideal sign that you would be OK independently is that you had one paper that was in an area different than your advisor's main research, that you proposed, and that you carried out with little feedback. This is relatively rare, though. However, if you can't point to a project where you both A) developed the central idea/hypothesis, and B) wrote the paper to a near-final stage without advisor feedback along the way, you might not be ready yet.

Second, the career value of both positions. I know of several people who have been very successful in highly independent postdoc positions, e.g. Lewis-Sigler Fellows at Princeton. However, those I know of who were the most successful ended up developing close collaborations with many senior people in their broad field. This does benefit you in terms of adding letter writers, and (sadly) in getting papers into good journals. It also creates a lot of benefit in terms of doing good science, too!

What worries me about your position (i) is that it may be a prestigious school, but could not be one of the best places to work on your chosen topic. If there are no faculty members working on similar things, not only do you lose their scientific expertise and letters, you also will not gain the benefit of things like speakers from that field coming to visit, conferences organized on campus on your topic, specialized journal clubs, etc. Those things matter a lot! 

How much the absence of support should affect your decision has to depend on how you mean that there are no people "directly knowledgeable in what I am working on". For instance, there are several very strong physics departments that nonetheless have no AMO or plasma physics research. If you plan to work on AMO or plasma physics, even as a theorist, I would not suggest going to one of those departments. The support structure and culture of a department matters a lot. However, if you are just saying that you work in general in a field X, and a subfield Y, and want to use technique Z, but there are no Z experts - maybe that would be OK. 

Also ask yourself: how confident are you in knowing what you want to work on? If that will change, the lack of immediately relevant faculty is also not as big a stumbling block.