Tristana has some really amazing ratios on her abilities (0.8 W, 1.0 E, 1.5 R) but she has a really good ASP steroid and her cooldowns aren't very good, aside from her ultimate which is a 60 second colldown. However, cooldowns can be solved by buying items like Morello's Evil Tome and Deathfire Grasp or Boots of Lucidity. Tristana has the greatest Attack Range at level 18, but she can jump pretty much just as far... I feel that AP Tristana is an effective Assassin style champion while AD Tristana is more focused for harassing or sustained damage in a team fight.. AD Tristamat (Tristana + Tiamats) can clear minion waves super fast and can be used for pushing lanes late game or keeping enemies out of the base in a losing game, or for killing enemies who stay very close together quickly..
Currently, I watch what my team needs, AP or AD, then choose my build at the last moment to pick up where we're weakest.