Hmmmm....
Now I guess that is a matter of taste... Personally, I decided to go to Pentax side because of its great FA 77/1.8 Ltd. I love to shoot portraits, candid ones or not. However, after shooting with several lenses I found out that the lens choice depends more on your shooting style than on what is generally considered to be a portrait lens... I suppose every lens can produce nice portraits (even the DA10-17 fisheye) with some restrictions of course, so I think that in order to choose the proper lens for you, you should firstly decide what is the result you wish to achieve with it... After 5 years of shooting I decided to sell my 77 and stick to 20-35 focal lengths for that propose with APS bodies…
*ist DS + DA16-45/4 at 16mm
*ist DS + FA77/1.8 Ltd
K20D + DA*16-50/2.8 at 21mm
*ist DS + FA31/1.8 Ltd
K20D + DA35/2.8 MACRO Ltd
K20D + DA*50-135/2.8 at 123mm
*ist DS + Tamron 28-75/2.8 at 55mm
MX + M50/1.7
MX + FA31/1.8 Ltd