Oly 75-300 or Panny 100-300 on a G3?
If you had an OM-D, it would be a tossup between the Oly and Panny - Oly lenses generally perform a bit better on Oly bodies, and vice versa. However, since you have a G3 now, I don't see any reason to go with the Olympus lens - you will struggle with the lack of IS, especially with the extra half-stop of speed that the Oly 75-300 gives up. Optically, I think the new version is the same as the old, so I don't think we'll see any dramatic improvement in image quality.
I'd get the Panasonic - it will work fine on an Oly body down the line (I've used mine on my OM-Ds with no issues), and you'll appreciate having the IS on your G3.