Glad you like the D3100.
The 50mm f/1.8 is a good lens for the money. So is the 35mm f/1.8 DX.
I wouldn't use either for portraits, myself. Both are too short. The 50mm might be good for photographing a person's full body while standing. The 35mm might be OK for whole body shots of groups, or for showing a whole person in an environment.
So what's the best lens to use for portraits? It depends on what you mean by 'portrait'. When I hear the word I think of head shots, taken in a space optimized for the purpose, and intended to make the subject look good. So most of what follows deals with lenses to take head shots of an individual, assumes you are not in cramped quarters, and assumes a shooting distance that gives resuts that are more flattering than accurate.
The wider the lens, the closer you have to get to the subject in order to tightly frame a portrait. And the closer you are to the subject, the more you exaggerate the size of the nose and chin relative to the size of the ears. If you move far enough away, distortion starts to work in your favour. The flattening effect of shots from a distance tend to produce more attractive portraits. A good distance to shoot from to avoid negative distortion and to provide flattering distortion is probably somewhere in the 10-15 foot range. Professional model shoots often shoot from even farther away. For the most accurate (but less flattering) depiction, you might want to get as close as eight feet. So for portraits, you want to choose a lens that tightly frames the subject at your chosen shooting distance.
I think the best choice of lens for individual portraits is something with an effective focal length (EFL) of somewhere between 100 and 200mm. On a Nikon crop body that means a lens somewhere between 65mm and 135mm. Besides focal length, the other attribute to look for is large maximum aperture - ideally f/2 or faster. This is so you can really narrow the depth of field.
So, did I just pull that 100-200mm EFL range out of thin air? No. It has to do with how your brain sees things and recognizes people. For a better explanation than I can give, see
Ken Rockwell's article about choice of focal length for portraiture. If you think Rockwell is a buffoon and 15 feet is too distant a perspective, consider that it still needs > 120mm EFL to get a tight head shot from the 8 foot perspective. That's > 80mm on a 1.5 crop. A common rule of thumb for portrait lenses is twice normal. If the 35mm is the closest lens to normal on a 1.5 crop, you're looking at needing 70mm. Another standard was 105mm on 35mm film. Again that yields 70mm on your camera. A lot of sources suggest 85-135mm EFL, but 50mm on a crop is only 75mm EFL. No matter which way you look at it, 50mm (75mm EFL) is too short for optimal head, or head and shoulder, shots.
The 85mm f/1.4 is wonderful for portraits (the 85mm f/1.8 less so, because its bokeh is not so good). Nikon also makes two specialized lenses suitable for portraits, designated DC, for Defocus Control. These lenses allow you to modify the quality of the out-of-focus elements of the picture. They are both f/2 and have 105mm and 135mm focal lengths. In a pinch, the 70-200mm f/2.8 can do a decent job if you can control the background, as can the 24-70mm f/2.8 at max zoom. The 60mm and 105mm f/2.8 micro lenses can also stand in for portraits.
People who recommend the 50mm (or something shorter) for portraits are usually thinking about the need for a fast lens (so you can throw the backfround out of focus), but are generally ignoring the perspective issue. Alternatively, they are thinking about 'portraits' as something showing a lot more than head and top of shoulders. Sample photos used to support arguments for the 50mm usually have a lot more in the frame than head and shoulders (and usually have it oriented for landscape, not portrait). Another reason the 50mm is often recommended is cost. Almost all the lenses that are truly good for portraits are expensive. On a crop body, the 50mm is
almost long enough, and is a lot cheaper than longer fast primes.
If by "portrait" you mean full body, or environmental portraits, then the 50mm is a fine choice on your crop body. If you are talking about taking pictures of friends in your living room, where you don't have too much room to move, again the 50mm is fine.
There are a couple of exceptions to the need for a longer lens for head shots. One is when the head is in profile and the ear is covered. (nothing to distort). The other is when distortion is a good thing. Babies and young children can look extra cute when shot with shorter focal lengths (you are used to seeing you own infant at close range), and you can get amusing effects from distortion when photographing adults close up.