Pure AD is not very good because TF lacks 1. range, and 2. enough skills that scale with ad.
Pure AP is also no good because TF's AP ratio has been nerfed again and again...
That is why hybrid is the way to go (nashors and rageblade and you will have both good autoattack and a nice amount of AP + CDR, afterwards you can transition to AD with one or two survival items then get bloodthirster/gunblade or Bloodrazor)
Most of my wins with TF are with hybrid builds, because AD is not very easy to pull off unless your opponents do not attack you(if you are dealing good AD damage, anyone worth his/her salt should focus you down first) and AP just is not as good as other AP casters.
As much as I hate Hannah Montana, I think getting the best of both world is the way to go on TF.