Personally both are correct to me. MKvsDC is not part of the MK story, it is its own universe. That being said it is also part of the MK series and therefore is an MK title just set in a different story. Its also the 8th Major entry. Unchained, Tournament Edition, Gold, Trilogy, UMK3 are all different editions of main entries to the series. MKvsDC is a main entry and can be labelled as MK8 if your just talking about the MK series as a whole. If your talking about the story then you disregard it. So MK2011 would be MK9 if your just talking about main games in the MK series. If your discussing story then its connected to Armageddon.
Wow that took a lot more words then I though it would!