That's a tough one!
Skirts are clearly better for showing off the legs, no doubt. Depending on the length, they are also a great indicator of how daring she wants to be. But, slutty skirts aside, they aren't so good at showing off her ass...
Slacks, jeans, are imo better for showing off the ass because the material and cut support and enclose the ass, inner thighs and crotch. But, they do not do a good job of showing off the legs from the mid thigh down...
I think I'm going to say slacks because Visible Panty/Thong Lines are far more commonly seen through them - especially summer weight slacks. And, there are few things hotter than a great ass walking in front of you, with VPL. :thumbsup: