Difference between Nikon DSLR and Canon DSLR?

So I'm planning on getting a great digital camera for Christmas in about a month. My parents want to know exactly which one to get. Does anyone have one or the other or have had both and can compare to two? I'm not planning on making them spend anymore than $500 so even used is fine. I've been looking at the Canon EOS Rebel T3i and the Nikon D3100. If anyone has these can you please give me some feedback on them. Or if you have another digital camera that you really like please let me know!

DPreview.com has a lot of information about cameras.

Often, Canon and Nikon will make cameras that compete with a specific camera made by the other company. And, in each case, it is possible that the Nikon is better or the Canon is better.

Also, it depends on which features you want. For example, I never use the in-camera metering or color balance, so it doesn't matter to me which has the better metering system or auto white balance.

I have two comparable cameras - Nikon D300 and Canon 5Dii. Yes, the Canon has higher image quality, but I prefer the Nikon in many situation because of the faster focusing, focusing under low-light, and wider depth of field. There's NO way that I could have learned that the "better" camera (Canon 5dii) is, in fact, not the best one for my usage.

One is made by Nikon, the other is made by Canon.

Probably not much difference performance wise. Both are excellent brands for dSLRs.