I don't know. I seem to like Greene's characters (so far) a lot more than O'Connor's, but I don't mind hanging out with O'Connor's characters for the length of a short story. Maybe O'Connor's characters seem more rigid to me?
Yeah, Wise Blood was a "difficult" (but long-remembered) novel for me, though I've enjoyed several of O'Connor's short stories.
My favorite reading lately has probably been Housekeeping by Marilynne Robinson and The End of the Affair by Graham Gree…