Crit Damage basically is an AD ratio. 13% Crit Damage gives you 13% AD bonus damage on crit.
So if you have 100% Crit Chance then Crit Damage Quints are better than AD Quints at 50 AD. if you have 20% crit chance you'd need 250 AD to break even.
For marks the points are even lower 42 and 213 AD.
So depending on how fast you can get the crit chance, Crit damage runes are better than AD runes.
However I have seen another calculation that I can't replicate right now saying that Armor Pen Runes add even more damage to a critical strike than Crit Damage Runes do.
Conclusion: never use AD Runes, nor Crit Damage Runes. always take Armor Pen. The only contender are afaik AS Quints.