Mixtapes are crafted according to a number of rules and guidelines, the most important of which is this: Your opening and closing songs must feel like opening and closing songs.
And that goes for Side A and Side B, though the opener of Side A and the closer of Side B must really feel like an opener and closer because they serve as the beginning and end of the whole tape.
Nothing is worse than a mixtape that begins or ends with a song that belonged somewhere in the middle of the pack.