I'd like either of these options:
a) Set an start and end time for the preview, effectively making short previews which showcase the best/most recognized part of the song. This is how it's done in most other rhythm games.
b) Have the loop start from the beginning of the song jsut like suggested by the OP. When the song plays tuil end it sounds better and makes more sense if it starts from the beginning, not from a point in middle of the song. Also, it does get highly irritating to have the song on the background start from a random point. Yes, I do often leave the game on backround while chatting on irc or surfing on the osu website for a while.