"Sidewinder" is considered to be the more challenging song to play on guitar. The main reason for this is the fast and intricate guitar solo that occurs towards the end of the song. The solo requires precise picking technique and a high level of dexterity to execute correctly. Additionally, the song features several riff sections that incorporate quick runs and complex chord patterns.
On the other hand, "Afterlife" is generally considered to be a more straightforward song in terms of guitar playing. While it still has some challenging sections, such as the main guitar riff, it does not feature the same level of technical complexity as "Sidewinder." The song relies more on power chords and catchy melodies rather than intricate fretwork.
Overall, "Sidewinder" is the more difficult song to play on guitar by Avenged Sevenfold due to its demanding guitar solo and complex riffs. However, both songs require a good level of skill and proficiency on the instrument.