Need Advice! Which one is best? Nikon D100 or Canon EOS Rebel XT ?

[youhou25]

Hi guys,
I want to get a new camera because mine is an old point and shoot (cybershot DSC-W1) and I'd really like to take it to the next level with a fully manual camera. I thought of the Nikon D100 and the Canon EOS Rebel XT (I saw both of them for sale used). Can someone help me figure out which one is the best and how much I can expect to pay for them?

Feel free to give me suggestions on other cameras too if you have ideas.
The max I'm willing to pay is around 400$, but less would be great. I want something fully manual that can shoot in low light settings with a wide iso range.

Thanks!