Responsive. Anything optimized for a phone (properly optimized, and I feel XF is) increases activity, conversion, and page visits. Source: I develop Fortune 500 mobile commerce websites, I need to know this stuff to do my job.
Users like a site that works well on whatever they're using without having to fight it. Right now I'm typing from my phone, so for me a site designed to work with my advice will always be better. My pick is for responsive.
(I'm also in the ecommerce business, albeit on a slightly smaller scale )