AD.
Here's why:
Her abilities scale from AP, however, she does not have nearly enough CC to be a mage. She will burst and die. The burst itself isn't even something to brag about.
The reason you play AD is because:
An ability that increases your attack speed.
A passive that increases your attack range.
3 additional abilities that are good as utility and not burst. One that cuts regeneration and helps you farm minions, one that allows you to jump long distances and slow enemies, and an ultimate with decent base damage and an AoE knockback to help your team.
Trust me, I see people try AP. They do 'decent' early game, think it's godlike, keep trying, fail, blame the rest of the team.
Edit: Oh and did I forget to mention she has the longest autoattack range after level 12?