Hey,
I have done a few portrait sessions and I must say that 98% I had the Tamron 28-75mm.
Unless you are going for the crazy bokeh, the 28-75mm will suffice just fine.
I suggest getting the 28-75mm first. After a few photo session you will see at what #mm you shoot at most. from there you can make the decision if you want 30mm or 85mm.
Plus the 28-75mm makes a really good walk around lens (most will disagree since they say 28 is not wide enough.. but for me it was perfect.. the 17-50//50 was just not enough for me)
But either way, out of the three you posted, overall i hear great things 
here's to pursue you :P
all tamron 28-75mm
28mm (widest) at f2.8
28mm at f2.8 - and they say it isnt wide.. psff!
38mm f2.8
71mm f2.8
75mm f2.8 (ignore the photoshop "dodging" at the bottom.. dunno why i did that)
heres an "eye" shot at 75mm f2.8. however it has alot of overdone pp on the skin.. dunno if its fair to post
http://www.flickr.com …/adrianvilcan/4372454999/
hehe.. hope this helps abit
