I have the 5100 and recommend it highly. I have many many cameras going back to the '60's (I am an ex pro and old) and it is my favorite camera of all time (other than the Nikon f2, a real dinosaur) including ones that cost twice as much like the Nikon d90, another great camera.
You can compare both the 3100 and 5100 but I would bet the 3100 is not any better. 24 mp versus 16 really is not an advantage. If you know what you are doing quality will be the same. Save the money and put it toward a flash or lens. The built- in flashes are not very powerful. On the other hand, more pictures are ruined by using direct flas than just about anything but a good one is necessary for many situations.
There are two kinds of quality: visual and technical. So 24 mp might be yield technically superior images but so what? (Actually, anything above 12 mp isn't all that different. A Canon g12 at 10 mp, for example, will rival any digital out there for great shots in 95% of all situations.)
