Either the 150 Sonnar or the 180 Schneider Tele Xenar lens will make a fine portrait lens. For individual portraits, I personally prefer the perspective of the 180 Schneider, because of the background blur that is more pronounced than compared to the 150 Sonnar. With the 180 lens, you can shoot at f/2.8 for ex&le at a distance of 10 feet, and have the background blurred out completely for a very nice perspective. Furthermore, with the faster speed of 2.8 with the 180 lens, I find this more versatile when shooting with existing light. With the 150 Sonnar, one can move in a little closer to get a similar perspective of the 180, but it is the background blur that the 150 lens cannot emulate.
As far as what speed one can hand hold either of these two lenses, this is something that will vary from individual to individual. Personally, I always strive to use a tripod when shooting with the 6008i. The slightest movement when the shutter engages will degrade the sharpness of your images. If you make a 16"x16" enlargement, you will really see the difference between a shot hand held, and the very same shot done with a tripod. I would estimate that most people could probably shoot with a 150 lens at f/5.6 at 1/250 of a second, and get fairly sharp looking 5"x5" prints. With the 180 lens being much heavier than the 150, doing hand held shots and obtaining a high level of sharpness is even more difficult.
If there is a way that you could see some s&les of the 150 Sonnar lens perspective, and some of the 180 Schneider lens for comparison, perhaps this would give you a better idea as to which lens you might prefer. Just to let you know, the 150 4.0 Schneider Tele Xenar lens will become available again sometime around October 2005, and this lens will be auto focus. The 150 Sonnar version is every bit as good as the 150 Schneider version, except the Schneider has a little higher resolution at f/4, and the Sonnar has a little higher resolution at f/5.6.