I've experimented with the same thing. I like both Logic and SoundTrack, but there's too much latency for studio work. I've recorded in GarageBand then exported the clips to either Logic or ST for effects and sweetening, then back to GB. It works pretty well for me. But I wish I could record in Logic or ST.
I think GB compresses the signal. I've recorded identical tracks and the GB files are always smaller than either Logic or ST.
E. Christina Herr & Wild Frontier - Lullabyes & Cautionary Tales