I'll run down the Basics of either arguement for you.
There are 2 sides. The first side claims Season 7 doesn't make sense unless Buffy's death by mystical forces and rebirth in Season 6, resulted in the slayer line once more running through Buffy. It may also continue to run through Faith, but Season 7 tells us the line now runs, at least through Buffy.
The other side of the arguement tells us "No. The slayer line runs through Faith and only Faith until Faith dies. All the inconsistencies in Season 7 when you believe this don't matter, because they were just the writers making mistakes. Joss and other writers have stated in interviews, the line can only run through Faith. That's all that matters.
I'll outline the intricacies of the arguements for you.
Slayer line alters in Season 7 camp.
* Joss made a statement in an interview where he said he didn't understand the problem some people had with Season 7, because it was "didactic in it's clarity". This is important I think, because what Joss appears to be saying is "believe what you see in Season 7"
* It isn't just Buffy who says the line now runs through Buffy in Season 7. It's also Dawn and The First, and it is perhaps implicit in the words or actions of Giles, Anya, Willow and Xander.
* We know the "Faith only" rule to be false, because at the end of Season 7 the slayer line goes all over the place.
* Beljoxa's eye tells us distinctly there is a "disruption in the slayer line".
* Giles tells us the "disruption in the slayer line" was caused by Buffy coming back from the dead. Anya tells us it was a result of the spell her and Tara cast.
* If the disruption in the line does not have something to do with Buffy, then why does the First want to kill Faith, and keep Buffy alive.
* There is no interview, or verifiable statement from Joss, or any of the writers after Season 6, stating the line can only run through Faith. Joss said it pre-season 6. Marti Noxon said it way before that, but I've never seen such a statement that would cover Season 7, nor to I believe one exists. Someone always says they know of one, but they can never produce it.
Only Faith is and can ever be the one true slayer arguement
* One of the writers (they're never sure exactly who) said (they can never tell you where) that only Faith could ever be the real slayer. They said this after Season 6. Don't ask for a link to the information, because if you do you're being a child.
* Everything in Season 7 that seems to suggests the contrary is just writer error.
* The fact that Season 7 makes sense if you believe the slayer line has been corrupted and changed after The Bargaining doesn't matter. It's better to believe the line can only run through Faith just because it is. It doesn't matter if we know the slayer line can be altered from Chosen. Ignore that.