100-300 lens choice: Pana or Oly?

Then why not get an FZ200?

Very similar price and weight to the 100-300. Second body reduces need to swap lenses and backup insurance when you go on long trips - take E-M5 with additional 100-300 and if M5 stops working the lens is useless, substitute FZ200 if M5 stops working you still have a camera. Uses same flashes as your M43. Electronic shutter no shutter shock problem so OIS actually works.

100-300 is essentially a daylight lens anyway. The superzoom should be fine during daylight hours too and the lens probably won't be too much worse than your satisfactory kit zoom.

On the other hand, the 75-300 (esp. the new cheaper version) makes perfect sense as it's observably lighter and more compact than the FZ200.