Lack of built-in flash on OM-D annoying or frustrating?

That is a little broad ...

I prefer the video on the GH3 because I can hack it for really nice bit rates, but maybe you want to run around and shoot hand held with primes so the Oly does that. The flippy screen matters to some people and there is no way to compensate for it. The flash is built in and can be useful, especially for firing optical slaves. There is also the matter of the grip. The G3 has one that is ok (G5 is supposedly better) while you pay a lot extra for a decent one on the OM-D.

If any of that matters to you, then the OM-D cannot really be called a better camera overall. Just better in several areas, while the Panny is better in several.

If you shoot fast action this could be useful. But who shoots fast action with CDAF

Only matters if you need it for video with primes. I shoot primes on the GF3, GH2, G5 and GX1 and have not missed IBIS. Your style nay be different but since you have been using a G3 for a while, I wonder ...

You could always get skilled at setting custom white balance. That skill is portable to any camera and will beat every camera's AWB hands down. Or you could shoot RAW and not worry too much.

If you like it, there is no substitute.

You can get one at extra cost for the Oly.

Oly likes to charge extra for almost everything (think grip, lens hood, flash) ... if you have the money, then there is no issue ... except maybe ergonomics of the UI and the flippy screen, which are not fixed by spending more money (but are fixed by spending less money on a G5 ) ...

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