Both can be done well, but I ultimately went with angels. I can't help but think back to Ogre Battle. Devils were awesome, but I really preferred Seraphims. It's hard to say why, though. I think I mostly identify with good, and angels are usually good.

Then again, I think that a demon portrayed as good is pretty weird, but also just as awesome as an angel portrayed as a villain. Now that I think about it, it's probably something I identify with even more.

But I still go with angels, since demons are very rarely portrayed as good.

Also, I don't see how Sephiroth is in any way a demon.