There is benefit in both, Q gives more damage and is better for dueling although W is much more useful in teamfights since it effects the attack speed of everyone thats hit by it. Q also only hits one enemy while W hits everyone in the line of fire. When going against an enemy with stealth such as Twitch, Evelynn, or Teemo, you can use the skill shots to still hit them and you will know if you land your Q. When landing your Q the cooldowns of all skills will go down by one second.

AP Ezreal is extremely mana hungry early game which is the reason for rushing chalice of harmony. The reason for Boots of Ludicity over Sorcerers Shoes is that Lich Bane will be applied more often. Rylias Scepter will make it much easier to land the skill shots. The reason for 2 points in Butcher is that Ezreal can not hit minions with Essence Flux and his Arcane Shift can not be aimed.