Olympus camera with Panasonic lens

I would not recommend buying the two bodies and returning one as that would be very unfair to the vendor who would then have a used body to resell. I would though, as was said, recommend you go to a camera shop or shops and handle both and see how they feel to you.

Take some shots while there with the lenses you mention and pay careful attention to focusing/ tracking speed which for both is reported to be slower that DSLR's for example. I have a Pany GH1 and an Oly E-M5 and I found the GH1 to feel very comfortable to hold and I am sure the GH2 and 3 are as good or better. The E-M5 however required me to buy the Oly grip to make the camera feel very comfortable. I do have the Pany 100-300mm and found the lens to be quite good for my needs. I have not tried to capture shots of birds in flight so I cannot attest to its ability in that endeavor.