Which Nikon Camera Would Be The Best For These Types Of Settings?

For a photographer who deals with low-lighting concerts for most of their work, would you recommend the D90 or the D5100? Why?

I know what I'm doing with cameras. I know that getting the D90 would be better in the sense I can use AF and AF-S lenses but I'm not really worried about that. I just really need to know which one will be have the best shutter speed and will give me the right amount of lighting for a low-light setting. Right now I shoot my concerts with the 50mm 1.8 but I'll be getting the 1.4 to use with the new camera.

Go for the D5100. Pictures and videos in low light turn out good as compared to D90.lesser noise.

For photographers who deal with low-light concerts, they get to have free access on-stage and perhaps even ride the crane. They get heads-up on when lights come out strong and what the performers will be doing. Heck they even get to shoot while they practice so the photogs have all the chances to get super sharp photos. The best part is that they're paid by the sponsors and producers of the show so they won't have to worry selling their pictures. So if you were wondering why you can't duplicate those fabulous concert shots you see in magazines and promotional items, you'd understand that it's beyond the camera, the lens or the skill anymore.

The DxOMark test results between the D5100 and D90 show less than a 25% difference in low-light sensitivity. And as indicated by DxOMark, this results in less than a 1/3 EV (1/3 stop) difference.

1/3 EV is a measurable, but not significant difference.

So go with the camera having features that you need. The D5100 is still an entry level camera, and the D90 is a mid-level camera. So the D90, even though it has an older sensor, may still provide some desirable features not found in the D5100 - such as the ability to use AF lenses as you pointed out.

At the same ISO setting and with the 50mm f1.4 lens all cameras will give you the same shutter speed in identical conditions.

No camera body will "… Give… The right amount of lighting… " since the lighting is dependent on the situation you're shooting in and the shutter speed is dependent on the ISO and aperture of the lens. The camera body is irrelevant except for how it handles higher ISO settings and for that the D5100 would be the better choice.

Learn about the Exposure Triangle - Aperture/Shutter Speed/ISO - here:
http://www.digital-photography-school.com/learning-exposure-in-digital-photography