I can't really recommend Critical Chance runes as they make your games very inconsistent.
It depends largely on how you want to play, but given that it sounds like you want to be aggressive early and get kills (which is a very strong choice with Tristana) then you would probably want to maximize your burst potential at around level 3 or 4.
Looking at the skills you will have at level 4, there is a mix of burst damage and sustained DPS from auto attacks. The basic combo is jumping on a target (lvl 2 jump: 110 dmg) hitting them with explosive shot (110 dmg) and auto attacking them approx 5 times with rapid fire active before they escape the slow.
So, looking only at the Marks and Quintessences (the offensive runes) and considering using runes for Attack Damage, Armor Penetration or Magic Penetration, we get the following values for 4 autoattacks and 2 spells:
- Armor Penetration - 471 damage dealt
- Magic Penetration - 335 damage dealt
- Flat Attack Damage - 465 damage dealt
The values are all calculated against a target with 37.4 armor and 35 magic resist, which would be standard for a ranged champion with Armor Seals at level 4.
If you calculate it again using average stats from a champion without Armor Seals and an armor value of 25, you get 421, 394, and 423 damage (respective to the list above).
The only conclusion that you can draw from this is that among Armor Penetration, Attack Damage, and Magic Penetration, Armor Penetration is better against enemies with less armor while flat Attack Damage is better when their armor is higher and both runes are better then Magic Penetration.
If you want to focus on last hitting then grab the flat AD runes, otherwise take the Armor Penetration for better late game and couple this with Armor Seals and Magic Resist per level Glyphs.