The fully electronic shutter gives you the fastest (20 fps) shooting rate - if that is important to you - and is entirely silent. The downsides are that you cannot use flash and the captures are only 12 bit.
For me, EFCS makes the most sense. It gives me a 1/250 flash sync and no shutter shock to induce vibration for macro or on tripod. And you get the full 14 bit capture depth. The only liability is that with wide lenses and fast moving subjects you can get some wierd distortions - but this is not the kind of shooting I do. You do hear the shutter, but that is the shutter closing at the end of the exposure, so this has no effect due to vibration on the exposure that just happened.
The fully mechanical shutter is like a DSLR. You get a 1/200 flash sync, 14 bit capture, but plenty of vibration. No distortion however if you are shooting fast moving subjects.
The following Blog has good information along with interesting test exposures and experimentation. The author (Robert Pollai) is in Austria.
He sees at least 2 stops of improvement from IBIS (at close distances) and as much as 5 stops at long distances.
Tom's Digital Photography Info / [email protected]