I would not buy either the 35mm or 50mm focal length primes as "portrait" lenses. The 35mm is too short a focal length for pleasing portraiture, and the 50mm lenses Nikon makes are no great shakes either at rendering faces in a pleasant way. A little bit longer lens, like the 85mm 1.8 AF-D would be a better choice I think.
^^^^^^^^^^^^^
This is all good advice.
This topic has been discussed quite a bit since the advent of consumer digital SLR cameras. This is what I think is going on:
When a person begins making photos with a camera and lens that affords variable apertures, they see the results of shooting with shallow depth of field. It's very appealing because there is separation of the subject from the background... an element of composition they had never achieved with a point-and-shoot camera. It looks totally different from what they're accustomed to seeing.
Then, armed with this little bit of knowledge, a common misconception emerged: The wider the aperture, the better a lens is for portraiture. Thus began the quest for a lens with the largest maximum aperture. You can often spot this by the phase coined and tossed around in an effort to sound like an industry insider: "fast glass." This merely means bright optics.
While shallow depth of field can be achieved using a lens at maximum aperture, it's not typically the best way of going about it. I can count on one hand the number of portraits I've made at maximum aperture.
It's far better to achieve this look by using a longer lens, which does give shallower focusing, AND moving the subject farther from the background. A longer lens will also give you much more versatility in choosing the background.
All this while preserving a more realistic perspective of your subject (not distorting faces).
Further, regarding differing senor sizes: The sensor size (or negative size) is considered the "format" of the camera. Each format has a "normal focal length"... a lens that shows approximately the same perspective of the human eye.
Dropping prices and improvements in auto-exposure controls put 35mm SLRs into the hands of a lot more people. Since the consumer market is tremendously greater than the pro market, digital camera formats are typically compared to 35mm film cameras.
Learn the format of your camera and it's normal focal length. A good starting point in choosing a lens for individual portraits is 2x the normal focal length.
I hope this helps.
-Pete