User blog comment:Kygehn/Wuju buy a Yomuu's?/@comment-4022742-20131225203902

"Critical Strikes" That is all. can critical strike, so he could buy either, but he doesn't auto attack a lot, so buying things like and  are generally better choices on him.

her skills can't critical strike, so getting is better on her.

It's really the same reason you don't get Critical Strike Chance on, and sometimes on. Going straight out damage on them with a is usually better, since their skills (or passive for the latter) Scales better with straight up AD.