depends on how close you prefer to be to your subject. i have seen very nice portraits from both lenses. i personally think the sigma to be more versatile, as you can use it for other type of shots besides portraits, whereby the 105 can be restrictive at times. Your call on whether you think you need that extra range and whether it justifies the price difference.Sigma 30mm f1.4 or Nikon 105mm VR?
Which is better?
i think that it has been mentioned before, the focal length really depends on your shooting style.
most would advocate the 50mm and the 85mm as the top 2 placements for portrait prime lenses on digital (1.5x crop), the 105 and the 35 are usually take distant 3rd and 4th placements. the only focal lenght with an available nikkor lense to AF on the D40 body is the AF-s105f2.8microVR (not the 105f2DC).
if budget was not a contraint, you could consider the AF-s28-70f2.8... but that'll be quite a stretch of the budget. else can also consider the sigma 24-70f2.8 or the 50-150f2.8... or get comfortable with manual focusing through the "tiny" viewfinder... :sweat: