they say the more mega pixels the larger you can print the image and you can crop the image more before the images starts to look bad. some people seem to think this is not true but I am not 100% sure if it is or not. I do know I can crop my images on my D5300 pretty heavily before they start to look bad. on the other cameras I had in the past I could not crop as much before the images starts to loose quality, I would have to guess that is due to the higher MP.
a image that is not cropped allot being viewed on your computer screen you will probably not notice much difference between the two cameras as far as image quality goes. as far as prints you will probably not notice much difference unless you are printing large prints.
if you take the D5300 vs the D7000.
the D7000 has more features like being weathers sealed, it has a focus motor built in so you do not need lens with a auto focus motor built in so you can uses lenses that don't have a focus motor in the lens. it has 2 SD card slots. I think it has high speed sync for using off camera flashes and some other stuff like that. its a much better camera as far as features go but as far as image quality goes it will probably be about the same as the D5300, just less MP
the D5300 has newer technology but less features. I think the D5300 does better with high ISO meaning better in low light. it has more mega pixels which to some people that might not be a big deal at all. it does not have the anti-aliasing filter which means slightly sharper images, they say they were able to remove that due to the newer technology used in that camera. it has a bigger buffer. it has wifi and GPS if you care about that kind of thing, it has a newer better processor and stuff like that.
those are some of the key differences between the two. you are looking at newer technology vs better features between those 2 cameras, its is a pretty tough choice IMO but I deiced to go with the newer technology.
the D7100 is the new version of the D7000, the D7100 has all the new technology plus all of the great features. you probably wont be able to find as good of a deal on the D7100. if I had it to do over again I would get the D7100 for all the features and newest technology. right now the D7100 is the best DX camera Nikon makes. their full frame cameras are better but cost allot more.
I find my D5300 to be a wonderful camera and I can easily live with out the extra features but I think they would be nice to have.