d5100 advantage not only lies with image quality.
It is
lighter and smaller
flip screen
cheaper (new to new)
probably much better liveview implementation, i think d90 LV implementation is really inferior to the newer d7k. LV-focusing definitely much faster than d90 as well
better video
If you want light and small, I believe m4/3 is the better system. I don't like a light DSLR body, as it doesn't balance well even with light lenses like a 18-105, and your pinky often gets strained when it falls under the front grip because it just isn't large enough. Flip screen is good in some situations, but the live view focusing is still not there, so if I wanted that I'd go with a Sony DSLT, which focuses much faster in live view.
If there is one reason why i take D90. Wider lens choices from nikon and 3rd party.
What do you guys meant by last longer?
Waiting for the price for D5100 now.....if it is around $1000 I am pretty sure D5100 is my choice!
D90 still going at $1600 but some of the used set looks very attractive (selling price, but worry abt high shutter count etc)
is a waiting game........