The electronic shutter is best with stationary or slowly moving subjects. The sensor “reads out“ data line by line, so there is a slight time lag from bottom to top of the image. When there is motion, that shifts the image slightly from one scan line to the next.

I use electronic shutter when I need to work in stealth mode — theatre, concerts, weddings, macro work, copy work (slides, negatives, prints, art, stamps, documents... where minimizing camera vibration is important).

Maybe one day, Panasonic will use a sensor capable of instant readout from all photosites.

If you do a lot of portrait work, an 85mm F1.4 is a real workhorse. In fact, anything from 85 to 135 is ideal. The advantage of an 85 is that you have tighter composition and this leads to better control of your backgrounds. If you use a 50mm, you generally need to move in a lot closer, and this will often result in an unnatural perspective on the subject ("horse face" narrowed and elongated, and usually unflattering), and lots of background that will distract from the subject. An 85 has less of this.

Zooming with feet changes perspective. This can work in some situations, but again, a 50mm can yield undesirable perspective effects when you get too close.

The 85 lends itself for tight portraits as well as full body shots. Longer focal lengths like 105 and 135 are also flattering, but sometimes the tightness of the composition requires moving back, which again, changes perspective, and faces become flat and wide. Comparing a 50mm to an 85mm you'll find that the illusion of perspective compression is greater in the longer lens, and when combined with skillful use of depth of field to isolate the subject, the results can be stunning.

There is a reason why pros use these focal lengths rather than shorter ones. The difference is a portrait vs a picture. Any lens can take a picture. A "portrait" lens does a better job.

As far as the two lenses you are considering, I have no experience with the Sony, but I've got a friend with the Sigma and I can attest to its awesome image quality, handing, build quality and value. Personally I've used the old Nikkor 85mm F1.4 AF-D and was extremely pleased. The Sigma is MUCH better.