For now strength is the way to go, Arp gets better later when you get more Arp to further buff your obliterate(top damaging skill when you get geared) and melee.
Arp buffs your Melee,Obliterate,Blood strike and necrosis(through melee)
Str buffs all your abilities through AP and synergizes well with unbreakable armor
Obliterate now is your 3rd damaging skill most likely, melee->FS->Obliterate or even FS->melee->Obliterate
So when you're low on Arp and can't even reach the cap it's a big loss in Str which is not worth it because arp performs better as you have more. good explanation here:
http://www.donorbashed.com/2013/01/a...ing-explained/
So go for Str untill you get at least 800 passive armor (most say 600 but IMO at that point you won't notice a significant increase)
and as for your confusion as on why most dks run full str even at bis it's mostly due to ignorance, FS used to double dip pre Icecrown making it very attractive and competitive vs arp but even then it was questionable, but now with Icecorwn it's no longer a question. Arp>Str @ bis.
So go for str for now and more importantly make sure you're hit(5% 163.87)/exp(22 is fine due to exp bugged atm) capped