Is neither an option ??
I find that for me, a legacy 300mm f4, paired with the Panny 45-175 ($300 these days) makes the most overall sense in terms of usability and IQ
Two issues I had with the 100-300 is that 1) it wasn't a very useful general carry for me because starting at 200mm equiv just left such a huge hole in the 100-200mm range and thats a range I came to really use a lot after years of working with a 70-200 on my DSLRs.
2) the times I did have a real use for the 100-300, was shooting at 300mm most of the time and it just isn't that great at 300mm. Not bad, but just not great mind you.
So basically it came down to the lens either being too long for general use, and then when there was a need for, not really having the IQ I wanted.
I tried the Oly as well, and really found the same thing, little better on the wide end, but also a little slower. That was on an EP3 so perhaps the EM-5 IBIS would make its slowness less of an issue on the longer end as its worlds better than the EP3 IS.
All and all though, its just a bit more expensive than I think its worth, and didn't really solve the issues I had even at hundreds more.
Solution for me was the optically very good 45-175 which is also such a nice compact IF walk around tele solution and at $300 very easy to justify. It even had a 46mm filter thread so pairs into a kit wonderfully with other m4/3 lenses. Just an all around great tele for travel and daily use.
Then for the random times when I do want reach, I use a legacy 300mm f4. For me, 600mm equiv means shooting wildlife, or more accurately put, zoo life lol. For those needs I typically value speed, as I'm often not shooting in great light, and I also value IQ, both of which a good quality 300mm legacy prime offers over a slower m4/3 zoom. Also they are rather affordable, not too heavy and very well built.
Might not be the best way to go for everyone but for me it made the most sense in terms of cost and IQ and overall usability.
Hopefully Panny or Oly will come out with a dedicated 300mm f4 or maybe a 400mm f4.5 "supertele" prime for sub $1000 and while it wouldn't be that small, I'd be very interested in buying something like that.