The D5100 has an excellent JPEG noise profile, very clean up to ISO 400 and despite some detail degradation from color noise quite usable up through ISO 1600. Beyond that will depend upon the content of your scene, though I wouldn't recommend ISO 6400 or higher. (Though you might be able to gain some latitude by shooting raw, I wasn't able to test raw processing performance as Adobe Camera Raw doesn't yet support the D5100.)
Canon leans just a touch more on the color noise suppression than Nikon, which I think produces slightly better results. It also helps that at equal settings, the T3i delivers brighter exposures, with slightly better white balance, than the D5100.
(1/30 sec, f3.5, AWB, ISO 1600, default color styles)
Night Vision mode is a very clever, useful way to take advantage of the sensor's capability of increasing gain up to ISO 102,400. In color, the results would be useless. But by converting the results to black and white, you get the ability to shoot in near darkness and obtain usable--though not optimal for high-resolution printing--results (scaled down you don't see the hot pixels). Autofocus only works in Live View mode.
The kit lens/D5100 combination produces some very sharp images, though there's more fringing than I like. And to give you a sense of how good the noise performance is, this shot was at ISO 400, higher than I ever like to go for sharpness testing. In this case necessary, because the D5100 exposes a bit darker than normal.
(1/50 sec, f5.0, matrix metering, ISO 400, 18-55mm kit lens at 26mm, standard picture style)
One of Nikon's less commonly found effects is Color Sketch. I don't really like its oversaturated pastel and sherbet color scheme--it makes me feel like I'm being attacked by an evil Easter hallucination.
I prefer the Neutral picture style; the others are too contrasty, which results in loss of shadow and dark midtone detail. (You can always increase the contrast later, but getting that detail back is hard.) However, Standard doesn't shift the colors excessively as on some consumer dSLRs.