Thoughts and opinions from those who have considerable experience with both of these lenses would be great. It would be fantastic if you have sample photos comparing the two...
Fisrt of all what sensor size are you using? There is quite a difference between a 44x33 sensor and a FF MF sensor.
Another important question is what other lenses do you have?
IF you use 35mm DSLR what focal length do you like there?
For a portrait photographer a good range of focal lengths is important. There are faces that really benefit from longer focal lengths.
While depth of field and bokeh are also important how big a subject nose is rendered compared to the eyes in more important.
See this link: Great focal length comparison by Stephen Eastwood.http://stepheneastwood.com/tutorials/lensdistortion/strippage.htm
Just remember this page is for 35mm DSLRs so do a focal length scaling to compare to MF, but be sure to consider any MF sensor crop.
On a full frame sensor a 300mm MF lens is equivalent to a 180mm on a 35mm DSLR
MF 300mm >> 35mm DSLR 180mm
MF 210mm >> 35mm DSLR 130mm
MF 100mm >> 35mm DSLR 62mm
The 100mm 2.2 is a lovely lens, but too short for most headshots or tight portraits. Just look at the 70mm in the Eastwood focal length comparison... 62mm would be half way between 70mm and 50mm
One really has to choose the right lens for the face you are photographing.
A damn good way of figuring out what focal length will be best for you is to shoot with a 70-200mm zoom and review your photos and see what focal range yo end up using and liking.
One issue you will have with hand holding both the 210 and the 300 is the lack of image stabalization and relatively slow lenses combined with modest high ISO MF performance. A monopod will be your friend with these lenss.
Another thing to consider when choosing the right lens for a shot is the mood you are looking for, especially for a relatively tight portrait.
A shorter focal length will give you a more intimate feel... "sort of like the view you would have approaching before giving a kiss"
A longer focal length will give more of an across the street or across the restaurant look ...."sort of she's lovely.. I better go over there and meet her"
Important choices when "telling a story"
Also for portraits I find tilt and shift (more shift) to be very important for shorter focal lengths... main reason why I like the Fuji gx680.