If weight and video are the features that are most important to you, then you should get the camera that best fits your needs. The D90 is a much better camera for photography as it's more customizable, and has important features such as a second control dial, AF motor, and wireless flash commander. But if none of that is important to you then it shouldn't be considered. If the D3100 has the video features that the D90 lacks, and that's what's important, then make the switch.