For exploration, either Comedic or Romantic.
For PK, Epic or Tragic.
The preferred bonus is not that big, so if you have a great comedic instrument, but are tragic preferred, use comedic.
Switching based on what you are doing during the log in is quite viable. You do have a cool down between switching, but it's not terrible.
Of course, this will mean keeping an extra instrument or two to suit the different rep.
Bard PK is a bit bland and takes some time to get used to. Just be cautious and know when to get out of a fight if it's your first bard.