What lens do you have now? Did you find an issue with it so you want to get another lens? I use 80mm on headshots almost 100% until i got 100 then it became around 80-90%, i don't think i will use 100 more than 80, even if i will do i feel that my 80 was a great lens and did the job, so if you have 80 and you didn't like i can understand, but if you don't have it then i may tell you go with 100, but i use that 80 on general photography as well and not just portraits, so i feel that 80mm can serve me more over 100, if i will shoot portraits outdoors i will go with 80mm because that FL is a bit wider so it may give me more room for another elements, and i may add 100 later, but for now i feel if i didn't have 100 i will not be missing anything, i may miss 80mm if i have 100 only and i tested 80 sometimes, i prefer wider side than longer even when shooting portraits, on my 35mm [Canon] i use my 24-70 more over 70-200.