The simple truth is that D80 and D90 are capable of taking excellent photos that will equal the quality of the D3100 and D5100.
The other truth is that the D3100 and D5100 are newer and so have newer technology that allows greater a bigger and less granulated photo in low light.
The D80 and D90 were designed from a more technical angle, meaning most settings can be altered with a push of a button and with a few extra technical features, so if you feel that you and your photographic itch are more technical and you will need quick access to aperture/ISO/shutter speed etc then its the D80/D90.
The D3100/D5100 is more consumer orientated as a entry level DSLR, with all the features on camera in the menu system, pretty much more technical steps are possible but will require a few more steps to accomplish ...
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