When I get in that situation, I look at what I am trying to target, and what the goals are for the site. Usually, the most narrow the niche, the more likely the traffic is a buyer. If you can get a "fat MILF playing with toys" niche listing for your free site promoting "bbw-milf-toy-world.org" then you really got something!
When in doubt, do the following:
Build a handful of sites promoting the same sponsor. Over a few days, submit some of them to MILF, some of them to MATURE, and some of them to REALITY. Watch where you get your traffic and sales from, and then target away. You have to measure before you can be sure!
Most link sites won't care as long as the site matches the cat. As an example, I have one submitter (who I think posts here at times) that has a tone of big tits material. In order to avoid overloading the big tits section, I sometimes move the submits to other categories that match the content, just to keep things balanced. So a big tit girl getting a cum facial might endup in cumshots instead of big tits that day. Either would be correct, and sometimes adjustments need to be made.
Play and find out!
Alex