Both are viable, AD would rely more on Attack Speed as well while AP would focus more on bursting with your abilities.

AD TF
Strengths: 1v1 fighting, damage vs champions, assassination
Weaknesses: Lack of range means no teamfighty. You'll get killed very quickly, and you have no AS/AD steroid like other DPS carries.

AP TF
Strengths: Earlygame domination, usage of all skills, Wild Card poking in teamfights, backdooring
Weaknesses: Less damage output in 1v1, building enough AP to backdoor means you will go down in 2 seconds if they teleport in.

At higher levels, AD TF is less useful due to people running away whenever they see the ult, or being more aware of the map/cooldowns and not walking solo when TF's ult is up.

hhhhh

AD is quite a impact and do a lot critical
AP does not really have any attack speed but have more power damage into spells
I Would say AD