I do not own either camera so I will not attempt to answer the first part of your question.
As to how soon do people want to upgrade just depends on the individual. The D3100 & D3200, even though they are "entry level" cameras are far more advanced than the best cameras of a few years back. Some folks just always want the newest model out regardless of need and will buy based on want rather than need.
I still shoot with a D80 which was replaced with the D90 which was replaced with the D7000. I buy based on need. I get much better results with my D80 today than I did when it was new. Why? Because I have learned how to use it. When my D80 will not achieve what I am trying to do, I will feel a need to look at other cameras and will upgrade. So it is up to you, enjoy your new camera for years to come or upgrade as the new models come out. Your camera will take the same quality of photographs the day you upgrade as it did new. (Providing no problems developed)
Jerry Jaynes Great Smoky Mountains of North Carolina