Taking photos in low light

The 500D is good for beginners, but also good for more advanced users. The bottom range of Nikons are more the equivalent of the EOS 1000D, EOS 1100D. The beginner Nikons are really just for beginners and as you progress the dumbed down menu system will become annoying. Don't forget the D3100 doesn't even have a built in focus motor, and so only a minority of AF lenses will autofocus (the newer AF-S or G badged lenses). 

All the older (cheaper) AF-D lenses would have to manually focused.

If it has to be Nikon, look at the D5000/D7000.

The Nikon D3100 has better low light image quality, it takes excellent quality photos up to 919 ISO vs 663 ISO for the 500D.

It's not the camera that will be the deciding factor here, but your lens. Get a fast lens with an f2.8 aperture or better, and both cameras will give you more than good results.