Which Camera will be great to buy?

I can buy either
Nikon - D3100 14.2-Megapixel Digital SLR Camera
or
Canon - EOS Digital Rebel T3 12.2-Megapixel Digital SLR Camera it

I'm gonna buy either one of those and also would be great for wedding pictures i will be taking in october so please tell me which one you think will be better. Some reasons why you pick that one.

Since you're comparing apples to apples there's no "better" because both companies make excellent equipment. I'd say pick the one you like bast for what ever reason.

Wedding photos? Either will work just fine.

I happen to be a Canon guy and only recommend what I own and therefore know so… Canon.

Either one is a great camera. It's a total personal choice here.
My personal choice would be the Canon T3. Why? The nikon D5100 is much better than the D3100. The biggest reason for me, however is that there's no auto focus motor in the body of the D3100 (or the D5100 for that matter.) Because of that you will have to purchase lenses with the AF motor in them in order for them to focus on the camera. You'll save money on the body, but spend it on the lenses.
How about the Canon T2i? I'd prefer that over the T3 by a long shot. I'd still prefer the T1i over the T3, but it's more a personal choice.