first, fire sells of the 5100 at 500 dollars which is a great deal. Brand new camera for 500 dollars.
it really depends on whether or not your want the older generation lens (mostly used), Nikon is quickly
phasing out these lens, and bring on the G generation lens.
on the otherhand, I really like shooting in manual mode (being too conditioned with old film cameras), so
I really like the front and back command dials, one for the aperature, second for the speed.
go handle both if you can, the 5100 is a bit smaller, I really like the way the D90 feels in the hand,
I have had both sensors, D5100 has the same as the D7000 and I really like the look of the pictures out
of the 16 Meg sensor. The D90 shares the sensor of the D300, which is quite nice.
so,, either way,, will work for what ever choice you pick.