In choosing a postdoc, how should I prioritize mentorship versus research opportunity?

I am an experimental physicist looking for postdocs in applied physics and materials science. In choosing groups to inquire about a position, I am trying to determine how to prioritize potential mentorship versus potential research opportunity. I see large "super" groups with huge numbers of postdocs and students. These groups have a high potential for research with lots of funding. However, I am worried about entering an ultra-competitive environment where I will receive very little mentorship from an advisor. I am wondering if choosing a smaller group will have an advantage because of receiving quality mentorship, despite the fact that the research topics might not be as leading edge or popular.