Does it really matter what flag reason I use?
It matters as when you choose a reason, you are reporting what is wrong with that post. If you are flagging a post as spam, it means the post is advertising something for which it is provided a link; you don't flag as spam an answer such as "I have this problem too; did you resolve it?"
Clearly, in the example I made, a moderator could notice it is not an answer, mark your flag as useful, and then delete the answer because it is not an answer. This would require the moderator to verify if there is any other issue with the flagged post, when the moderators are supposed to just verify the flag is valid, and take the necessary action when required; in fact one of the reasons for declining a flag is, "a moderator reviewed your flag, but found no evidence to support it." While in the example, it's quite clear the post was not an answer (and the flag was someway useful), it is not always possible for who review the flags to see there is another problem with that post. That is the reason why we select a flag reason; differently, if the flags served as "look, something is wrong with that post; discover what" purpose, there would not be any reason to choose between.
A moderator could decline a non correct flag, even when it helped to discover a post that needed to be deleted, when a user keeps selecting the wrong reason when flagging posts. In this case, the reason is avoiding the user keeps doing something that is undesirable, or to teach the user what should not be done.
In your case, if you have a doubt between two flagging reasons, first be sure to know exactly the purpose of those flagging reasons; if then you still have any doubt about which one to select (which should really happen only in few cases), then choose the one you think more appropriate.