Prefer iPhone when synchronizing playlists between iTunes and iPhone
Solution 1:
I'm unaware of any settings that this will work with iTunes/iPhone out of the box. But you could give iGadget X a try:
** iPad, iPod touch and iPhone Support **
iGadget (Windows and Mac) now supports the iPad, iPod touch and iPhone. However, due to the limitation of the Notes feature on these devices, only the Music Transfer, Playlist Transfer and Music Listing functions are available when used with the iPad, iPod touch or iPhone.
PS: It's shareware ($19.99)