Comparing both of these boards the plus points of z87 killer are
-Better audio chip 1150
-Supports three way crossfire
Plus points for z87-a
-Asus is a more reliable brand
-Legacy pci slots (if you need them)
The z87 killer also has killer lan chip which requires additional driver installations and hardly makes any difference.Asus boards are said to have better quality components.Other than these you can also consider the asrock z87 extreme4 and msi z87-gd65 boards.Asrock boards do support fan control and single bios isn't really a bad thing.Customer support depends from time to time ,you can read newegg reviews to get an idea perhaps.Asrock does have gold caps and i am sure asus has equivalent or even better components on their board.I have seen asrock's 555 usb boost and wasn't really impressed .