105 is more "conducive" to 3/4 length portrait on FX. But then you can just step backwards a few steps and get a similar POV for 200. You should try setting your 70-200 at 105 and at 200 and see if you like the results you get.
If you are mainly going to use the lens for portraits, why not buy a portrait lens, like the 105/2.0 or 135/2.0? For manual focusing, portrait lenses usually have a focus throw that is much better suited for portrait distances. I use a 105mm f/1.8 AIS. Here are a couple of samples, all street shots with ambient light. They are taken with the S5, so it's DX format:
