Basically, there's really no effect on image quality from adapters. Only changes in performance.
For portrait shooting - I have a lot of opinions, to be taken with a grain of salt (but I do spend 90% of my paid shooting time doing headshots for models / actors / corporate clients - other 10% is theatre promo/production). Most people will steer you toward the 85 G Master. And with good reason. It's f/1.4, has VERY nice creamy bokeh, very sharp.
I'd also suggest at least taking a look at the Zeiss Batis 85. It renders a little differently. For my uses, I actually prefer it over the 85GM. I think it has a little more punch, great colors and contrast right out of camera, whereas the 85GM (although creamy as hell) has a slightly flatter rendering and requires a little more coaxing in post to get the same sort of punchy 3D quality I see with the Batis out of camera. I may be in the minority on that opinion, but worth looking at IMHO.
EDIT: It also sort of depends on what sort of "headshots" you do. If you actually do true "headshots" like I do, as in head and shoulders or waist-up at most, the Batis is perfect. I think though, if I spent more time doing more environmental portraiture (rather than actual headshots), with more focus on surroundings, more 3/4 or full body and longer distances to subject (like the sort of stuff Vin shoots), I'd probably lean toward the GM... so that when needed, you can really make that background melt away, which is harder to do with the f/1.8 Batis, which can tend toward nervous busy bokeh with some backgrounds (particularly like golden hour sunset coming through tree branches type settings)
