everyone one is right about pattern density and 6 will have more holes in the paper. but if you look at distance and ftlb of kenitic energy retained by per pellet to achieve a broken neck bone, then you have a different story.
but of course pattern density will again play a part, cause 2 #6's will hit harder than 1 #5 at any given distance. if i remember correctly i believe it takes 3.5 pounds to break a turkey bone and a single #6 pellet loses that past 32yds. a single #5 is like 42yds.