Why does the Darkness have a specific family name and lineage, and have to reserve its energy for 21 years in an Estacado whilst the Angelus can switch hosts whenever she dies? Shouldn't the Angelus also follow the family line and 21 years rule too? whaddya guys think?

The difference I believe between the Angelus and Darkness is that the Darkness powers are not really the Estacado to begin with...they just have access to it. The Angelus on the other hand is the real deal, it can chose who it wants to take over and must be contained in a female vessel that has enough power to contain its power... the first Angelus that Jackie killed was actually one of the best, having killed most of Jackie's lineage.

The angelus also gains control of its vessels mind and its host can not really act on her own but top cow is somewhat changing the entire landscape since the balance has been broken. If Danielle or Sara gains the Angelus, their background as the witchblade can give them more control over the angelus and thus, their consciousness can have the dominant personality over the wielded power.

Personally with all that's going on in the Darkness at the moment, I've come to think that maybe it isn't just Jackie's family line, but a few unique family lines that wield the darkness. Like remember the issue when he meets the guy that teaches him that Sovereign is lying to him about the split between his body and soul. If that man was a former wielder well maybe it's in his family bloodline as well. That would make sense because if the Angelus did manage to kill the darkness during it's awakening, then technically she's only killing off that bloodline and it's access to the darkness. Others would be The Darkness covering it's bases. 
 
As for the differences between the two, well I'm thinking it as matter of open warfare versus Guerrilla tactics. The Darkness can be subtle and sneaky and the Angelus has to work just that much harder to find it and stomp it out, mean while the Angelus is so bold and arrogant that it welcomes all comers. It's two different looks at the same kind of situation and the kind of answers you'd expect from the two forces and their opposing philosophies.