Which one would you suggest for a beginner in photography?

The canon EOS rebel xs or the nikon d3100?

I might be a little biased but I like my Nikon D3000 but the D3100 has video and I chose my D3000 over the Rebel so I would go with the D3100 because it has video. Either way, you're getting a good camera.

They are both good cameras but I have always owned Nikon because they make their own lenses

I think these cameras are competing models in the same price range; you can probably expect similar performance from both of them.

If you are learning photography, you should know how to change your shooting mode from manual to aperture-preferred, to shutter-preferred. You should know how to adjust your white balance and your ISO settings, and how to shoot in Camera Raw. You should know how to change your flash settings.

These are the most important things to know about camera function and shooting. Make sure that you are comfortable with the menu settings and the ease of finding the buttons that will make the switches for you. There's nothing worse than having a camera that you are clumsy with, that you have to change the settings on the fly and you find the functions to be counter-intuitive.

The size of the sensor is important, but as a beginner it's probably not on the top of your list.

Both Canon and Nikon make excellent lenses, and I think both models you mention allow you to interchange them. You'll find Canon vs Nikon users to be like Republicans and Democrats, or PC vs Mac users - those who like their choice will swear by it, and confuse you more.

I have heard that the "Kit" lens (the cheap lens that comes with the camera) for Canon is sub-par, it's plastic rather than glass, and isn't extremely accurate. I've only heard this - and the same may be true for Nikon but I don't really know.

Whichever one you choose, get used to the mechanics of it and make sure you really know what you're doing with it, then invest in a better lens.

Honestly, they are both great cameras. The D3100 does have a guide mode that can help you learn how to use it better, but in the end the choice will really boil down to which camera you feel more comfortable with.

Have you gotten a chance to play with both in store? If not, you really should take a bit of time and compare the two. Is there one that feels more comfortable when you hold it? Do you prefer the menu system on one over the other? Take an SD card with you if you can, and ask an employee if it's okay to snap a few shots in the store. I've always been given permission when I asked, and taking the images home and comparing them side-by-side on my computer helped me narrow down my choices when I was looking for my DSLR. Trying them out for yourself is really going to be the best way to choose between the two models.

Edit: What "point5" has said about the Canon kit lens being plastic rather than glass is absolutely incorrect. The optical elements inside the lens are glass, but the lens CASING is plastic rather than metal. The same's true for several of Nikon's lenses, too.