Which should it be?
They could simply be tag synonyms, and there isn't any need to delete one (by merging).
I don't think it really matters which one is the master and which one is its synonym, and having the option to create synonyms means we can keep both of them, have users that will be adding them to their questions decide which one they prefer, and have them both work in the same way as far as filtering questions by tags goes.
Alternatively, we could merge them and decide on a single one to use, but I don't think there's really any need to abolish the use of one spelling of the tag, when synonymizing them is as straightforward as in this case, one merely being an acronym of the other.
I went ahead and made the extra-vehicular-activity be a tag synonym of eva. You can test them out how they behave by clicking on each of their links here, or by opening tag synonyms page on [tag:eva]. ;)