TTFAF is harder than Raining Blood. But I like TTFAF because it is a bit of a challenge to me(Currently trying to FC it on Expert, only got 3* :( ).
Hardest parts i think are Red Snake(the hard tapping part at the end of the solo), The " What the!!??" section and Sam's solo. The intro is the most easiest part on the whole song to learn, people are wrong there the fact that it is the hardest(Though it's much more difficult to squeeze The Star Power), otherwise the whole song is easy. It's not much of a pain. Well, the riff is quite hard to squeeze. Otherwise, it's easy to get a hell lot of points. RB is harder, the fact that the solo is a pain in the *** to squeeze, and the intro sounds like a cat getting strangled.
"Close your mind,
and
Open your heart."
-Swedish House Mafia