I have the 100mm macro and it is a lovely lens but I find it too sharp for portraits - all the skin pores are on show! Also, at f4 it is a bit slow for available light shots. I like the 85mm 1.8 and also use the 135mm 2.8 which is relatively cheap and a nice lens to use. At f2.8 it is not so sharp but has a nice softness which really suits portraits.
The vivitar 90mm 2.5 is also a great lens and I find it is quite soft at f2.5 which makes it nice for really close up shots of eyes and mouths etc. However, get to f5.6 and it is too sharp again. It is also quite a big and heavy lens and though it feel nice to hold, after a while it is a bit heavy if you intend to do lots of candid portrait shots.
Just to throw in another idea - get an fd / m42 adapter and use some of the incredible super takumar (pentax) lenses with stopped down metering. The 100mm 2.8 super-tak is a brilliant portrait lens as is the 85mm 1.8. M42 lenses are really cheap compared to FD and are (in my experience) amazing pieces of glass.