“One site does recips with the links, and the other receives all one-way links.”
Not a fair test, since the PR of the one-way site will be significantly higher since it will not be linking out. Also, it won’t have the benefit of linking to other sites on the same theme. In other words, there is more going on than just the actual type of linking.
Their is nothing unnatural at all about reciprocal linking. As always, when people who don’t get seo latch onto an idea (oo,oo, getting links is good) they tend to go way overboard and make these monster link compost piles… three pages of content, hundreds of link pages. This is not natural because the linking isn’t the same as what is basically the non-seo dictionary definition of “linking”: you link to something because it has a point of value to a person on that site or that page. Recipricals between related sites is not just normal, it is inevitable and vital. Trash reciprocals are just trash. The point is, do what is normal and sensible and useful, and don’t do the trash.
Reciprocals are just like pages on your website. It’s normal and good to have pages. It’s trash to add hundreds of thousands of doorway/keyword/”click-here” pages. It’s not the idea of a thing that is a problem, it’s what you do with the thing.