I use them both. As of late, I been favoring the 15" coil. It is like a vacuum cleaner picking up the crumbs and find it goes deep especially on dimes. After a while, I now find it just as easy to pinpoint and separate good targets from nearby junk targets.
I will mostly use the 15" after I used the smaller coils and picked out most of the trash. Although the 15" it is a bit heavier, I find it very well balanced and sort of slows down my swing which is a good thing.
Both the 6" and 15" are great tools for their appropriate applications.