Which one should i choose the sigma or the nikon?

My birthday is tomorrow and my mom want's to buy me a sigma 70-200mm f/2.8 ex dg hsm os or a nikon D7000 currently i have the d3100, 50mm f/1.8g, 55-300mm f/4.5-5.6, and the 18-55mm kit lens.

Go for the Nikon… You'll get a upgrade in terms of sensor and image quality too and some good quality optics too…

I'd get the lens. The camera is an upgrade but the D3100 is a good camera and upgrading won't make as big a difference as a good lens. The lens will last you a lifetime while new camera bodies will come every couple of years at worst. Get your good lenses and when you are really ready for a new body, get the latest one. There's a big difference in a 70-200 f2.8 and the lenses that you have.

After you buy a DSLR, the next thing you should do before upgrading it is to buy your lenses. The best camera in the world is no good without a good lens to go with it.

The only exception would be is if you want to use Nikon AF lenses - as there are some really good ones - but they will not autofocus on your D3100. You need the D7000 to autofocus those lenses.

I'd take the sigma lens over the D7000… But that is because i know that glass is more important than body…

what do you think a D7000 will give you that you can't get from a D3100?

i suspect you haven't worked with the D3100 enough to NEED to upgrade and think that the D7000 will impress your teen friends and that you'll take better images… But if you suck with the D3100, you will with the D7000 as well…

Use the money to pursue subjects: All the gear in the World won't help if what you're taking a picture of is a front yard.
If you spend too much time focusing on gear, you won't get the pictures and they won't be as good.
Oh, and if you have to miss one opportunity for photos to get a piece of gear… Don't get the piece of gear.
To elaborate:
D7000 is due to be replaced by a "D7100" I think, wouldn't you feel silly then?
But seriously: I have the D3100, and I've found that it's probably going to last me 5 years, unless I break it or something. The only thing it doesn't do is exposure bracketting. Makes HDR setups annoying. But other than that, I would never spend money on a camera body(what you'd call the camera) instead of getting a better lens. I would love to have a 70-200 f/2.8 on my D3100! The pics would be amazing! Well, you know, if my skill level was adequate. Which it's not. But I'd have fun with it, and that's the point! But the other thing I see here is that you already have 18-300mm focal length covered. I honestly don't think you need the 70-200 either. The only really good reason to get that 70-200 is if it has image stabilization and the 55-300 does NOT. Otherwise:
You need to use that money to pursue subjects.
The hardest thing in photography, in my limited experience, is finding truly interesting subjects.
And that money can disappear very quickly with costs associated with the travel and such needed to find those subjects.