just calculate it through.
As ashe you will most of the time be attacking enemies with not too much armor (let's say 50-100)
If you reduce ur enemies armor to 0 every point of armor pen is worth about 1% more dmg.
(Someone with 50 armor receives 66% dmg, bringing that to 100% with 50 armor pen let's u deal 50% more dmg. Someone with 100 armor receives 50% dmg, again ignoring this will give you 100% more dmg)
If you have got too much armor pen it will become less useful by the factor you've got too much (having 100 armor pen against 50 armor will make half ur armor pen useless)
If you have too little armor pen it will become slightly less effective (50 armor pen against 100 armor will increase ur dmg from 50% to 66%, by 33% so you are at 0.66% per point)
Now a mark will give you either a really small amount of attack damage (will increase ur dmg by something like 0.2% per rune at lvl 1, decreases during the game. Also boosts ur volley and passive), 2 armor pen (ideally meaning 2% more dmg, scales well with more armor pen, stays useful if you don't. Also boosts ur volley and passive), 2% attack speed (not sure, would give u 2% more dmg IN THEORY and gets weaker the more aspd you stack, does not affect volley) or 1% crit chance (gives you 1% more damage throughout the whole game, get's less effective if stacked with crit chance, gets more effective if combined with crit damage, does not affect volley)
So as you can easily see armor pen runes will most of the time be the best choice.
They are also good as quintessences (purple runes) but the AD ones aren't too bad either here (or defensive ones like flat HP).
Only against high armor targets crit chance is actually more effective. (if you have a 200 armor target and ignore only 50 it will boost ur dmg from 33,33% to 40% , so by 0,4% per armor pen. So here an armor pen rune will give you 0,8% more damage, a crit chance rune 1% more.)
PS: If you're wondering why I chose 50 armor pen as an example: 18 armor pen from marks, 12 from quints, 6 from masteries and 15 from brutalizer is about 50.
And one more word on aspd: aspd is cheap and easily obtainable from items, don't waste ur runes on it. (Plus as a kiter you don't benefit from aspd as much as from damage)