Depending on the camera you're shooting, there could be a difference, but most people won't notice. It does not affect DOF, but it can double the shutter speed which may reduce motion blur.
You should always shoot at base ISO of the camera, whatever that may be, for maximum dynamic range. So, if you have a D3, which has a base ISO of 200, you should use that for your portraits, if you have a D3X, which has a base ISO of 100, you should use that.
On the Nikon camera's, that start with 200iso and go up till 3200 or 6400, this range is made in the analogue part(sensor + amps) and the lo and hi settings(100 and 6400, 12800 + 25600) are made in the digital part(processor).
