I see both sides of this;
• On the one side you have pretty much identical casts and it focuses around two teams of 'Avengers', so I can see the justification of believing Civil War to be an Avengers film. Some cast members have even referred to this as an Avengers film at one stage or another
• On the other side though it depends how the film itself chooses to focus, if it focuses intently around the character development and actions of Captain America then it would be more of a Captain America film. Even though the cast encompasses the Avengers, they may not be the focus
Personally I think it would have been a lot easier simply referring to this film as Civil War, rather than grouping this with the Captain America series, mostly because the prospect of Civil War didn't really fit with the direction the Captain America films were going. While the series was focusing on him tracking down Bucky and defeating Hydra, its now changed more to him facing off against modern day political groups that have caused a split in different factions.
Either way you view it is fine as there is not really any right answer...