My D5100 is lighter, smaller, better in low light, has higher resolution and dynamic range, has newer, constant AF in video. I've handled a friend's D90. It has a better viewfinder, less menu diving for setting changes, and what feels like a heavier and better build. The D5100 is now behind the D5200 and the D90 is back two generations from the D7100 and D7000, in comparable class. Here's another way to look at it: the D90 performs no worse now than when it first came out as a very respectable camera. If you can find a clean one with low shutter count it could serve you well for a long time. I like my D5100 very much, understanding that it's designed for people with smaller hands who aren't into a lot of manual control. That describes me. You might try handling samples of both models to consider which is the better fit. Both are good!