Which camera do you have?
If you have one with CLS built in then get the 600 or 800 since you can use them wirelessly.
If you don't have CLS built in or you have an older model Nikon go for the SB-80 since it is cheaper and you won't be paying for features you can't use yet, besides, it will still work perfectly with newer model cameras. The 800 is a faster flash and puts out more power in repeating flash mode than the 600, and the 800 can be used as a wireless flash slave and commander while the 600 can only be used as a slave. The 600 lacks a PC sync for pocket wizards and other "off camera" accessories (this may be important since you are doing portraiture work). The cost difference is a bit high ($100 - $150) but you will see why when you shoot with them both.
Thom Hogan (linke
http://www.bythom.com) actually writes books about the equipment he uses, in detail.