Which brand is better when it comes to DSLR cameras? Nikon or Canon?

Here's a particular example: Canon EOS 450D V. S. Nikon D3100
The point of my question is Why do other people prefer to buy CANON even if it has lower specs than Nikon. I wanna know why.

I'm gonna say Nikon.

Well, partially why you think the Nikon has better specs is because you are comparing a camera that is ages old, to a new camera that is of a higher tier, of course specs are going to be different.

Nikon and Canon are considered equals in nearly every way. Its like coke or pepsi, NVidia or ATI, AMD or Intel. You will have fans from both sides, and both are quality products.

the Nikon D3100 would be more equivalent to the Canon T2i (550D)

Everyone has there own opinion on them. They are equal just depends on what you like and what you want. Choose the one that pleases you more. Thats how i see it anyway.

All manufacturers sell different levels of cameras just like cars.
Low beginner snap shooter.
Medium advanced snap shooter
High Pro photographer
1. Auto focus built into the lens for faster focus.
2. EOS EF lens investment for my older EOS 35mm body are fully compatible with newer EOS digital body purchased.
3. Magnesium alloy camera body.

Which brand is better? Depends on people's need.

for that comparison - Canon EOS 450D V. S. Nikon D3100
id go for nikon because it's newer and better specs BUT i'd probably buy the canon. See below for my reasons.

First reason is lens autofocus, the nifty-fifty 50mm($100) will autofocus on all apc canon dslr but not on nikon 3100/5000 (and some nikon brands).

next reason is when people have significant investment on Canon/Nikon lens/flash/gears.they'll be leaning towards either brand.

Canon vs Nikon are 2 best brands on DSLR. People choose either because they like the other brand or already have a significant investment on the brand. Sometimes the warrant/service is better in their area.

Me, personally, id go for canon SIMPLY because there's a canon service center right outside my office.nikon service center is so far away, i have to call nikon and ship my dslr to them. It's not accessible by car.

Between Canon and Nikon, there's no better. Brand choice is affected more by brand loyalty if not the perseverance of the salesman. If you want better than the two look at Mamiya and Hasselblad.