I have the Nikon D3100. It came with the 18-55mm lens. I shoot mostly portraits. Love doing maternity, children etc. I am buying one new lens soon and am not sure if I should get the 35mm AF-S f/1.8G or the 50mm AF-S f/1.8G.
Both of those lenses are excellent choices for your next lens. I don't think you would be unhappy with either one. The definitive answer is something that only you can know though. Luckily you have equipment that allows to you shoot at these focal lengths already, so simply test out the kit lens you have at approx 50mm and approx 35mm and decide what suits your style of photography better. As another user has mentioned, you can also look at your past images and decide based on that as well.
Beyond that, know that virtually all portrait photographers will use both 35mm and 50mm focal lengths, on both APS-C sized(D3100) and full frame sized sensor cameras. As lenses get wider, they do experience added distortion, so that is something to consider. But it isn't practical in most locations to shoot with a 100mm lens to virtually eliminate distortion on an APS-C sensor indoors, so you just have to decide if 35mm or 50mm gives you distortion that you are comfortable with or not.
Personally, I would go with the 50mm or even the 85mm 1.8
The perspective you get from the longer lenses is generally more pleasant for portraits than the wider ones. 85mm is a fantastic focal length for portraits on a DX camera.
Also if you move up to FX those longer lengths are still usable for portraits.
35mm is a very good all around focal length for a DX body, but if you specialize in portraits I think you're better off with the longer focal lengths.