When in doubt, put AD mark.
With or without interest, raw money is always good.

It depends on the situation. If your Darius it would be dumb to put Arm pen on him because of the passive from his E. Also you need to consider if your champ is a good late game champ or good early game champ. If their late game is good then definately Strength is the way to go to help boost you enough to get through early game. If your late game suffers then yes Arm pen is the way to go or if you need the extra armor penetration because your champ already has enough AD.

i lold at sam quinee, who cares armor pen is as good as these, sometimes more usefull, especially lategame ( i am pro)

ARMOR PENETRATION MARKS VS FLAT AD MARKS, The common flamewar i hear daily

Can be useful for champs, that can deal magic and phisycal damage, depending on bonus AD or just AD. I like this rune more than penetrate runes.