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.