I would probably go for less searched for keyword combos but have quite a few listings and make the ad specific to each of the keywords you want to be found under - not just some general free porn pictures title when somebody searches for free asian sex pics.
A lot of the ads on there aren't relevant a lot of the time for what the surfer is looking for so if you are more specific you stand a better chance of standing out and getting a targeted clickthrough
Use the Overture suggestion tool for keyword combos.