ack!! i dunno that FLAC have larger filesize...
sigh, im sticking with mp3 (galaxy s2 got 16GB left, n8 left 8GB)...
If you're planning to listen to it with your Galaxy S2, then I would definitely consider using .ogg instead of .mp3 (I know I mentioned it before, but it's worth reiterating). Android is very, very good at handling ogg files out of the box - it treats them basically the same as mp3 - so there's no reason not to use it. Unless your collection is entirely mp3 already anyway, in which case I can understand not bothering to change it unless you're running out of space.
Basically, as said, you're not going to hear any difference between ogg and mp3 as such, but the compression used by the ogg format is significantly better, so tracks will take up literally half as much space, or less, at the same quality level.
If you're planning to re-encode any of your music to mp3, you may as well just encode it as ogg instead. If your music is in mp3 format already anyway though, then there's probably not too much point in going to the effort of reencoding unless you need the space.