All things (size/cost/noise/af-speed/DR/IQ) being equal, I will pick the m43/f1.8.
All options have similar depth of field/bokeh, but the m43 sensor will have more light (f1.8) for a similar shot. This will give you better IQ.
But life isn't that fair. M43 ISO performance still falls behind the larger sensor. Dynamic range at different ISO also matters a lot.
My main criteria for switching a system will be lens availability. I've been tempted to switch over to an A7 for awhile, but the lack of (affordable+fast) lens does put me off.
Every system has its advantages, and my current Nikon setup (I shoot nature, legos and family) provides me the best lens selection (for now)