They are both lossless formats so it doesn't really matter which one you use - I would say use alac if you have to because you are using an apple product else use flac just because it's supported more widely.
FLAC has a better compression ratio so it will make your filesizes smaller. If you are trying to save space use v0 mp3s instead.
see here: http://www.head-fi.o...rst-impressions
EDIT: Answer: there is no 'average filesize' as the filesize differs depending on the length of the song - so unless you want to take a sample of the length of every single song ever made there is no correct answer to this question.