I'm thinking about upgrading my camera?

I currently have a Nikon D3100, which would be better a Canon T3i or a Nikon D 3200

Added (1). I have 3 Nikon lenses with my Nikon an a Canon Eos rebel K2, 35mm format with 2 EF-S lenses

Added (2). I have 3 Nikon lenses with my Nikon an a Canon Eos rebel K2, 35mm format with 2 EF-S lenses

They say Nikon D3200 is a good choice. However, it depends to what features fit to your need.

How much of a Nikon system do you have? Several lenses? Or only one? If you have only a single lens the change to Canon may make sense to you. I'm a Canon guy and I really enjoy their gear.

However, if you have a larger system as in some lenses and other Nikon gear it would make sense to stay with Nikon.

Don't misunderstand, both companies make excellent gear but why try to sell all your Nikon gear to change systems unless you have money to burn.

What can the D3200 do that your D3100 can't do? What can the T3i do that your D3100 can't do?

If you're dissatisfied with the results you're getting with your D3100 its probably due to you, not the camera. Repeat this 100 times: "It isn't the camera its the photographer."

The results you'll get with any camera are dependent on your knowledge of Light, Composition, Exposure and Depth of Field and your skill at using that knowledge. If your pictures have poor lighting or poor composition or poor exposure and depth of field is an alien term then no camera will magically correct those defects. Only study of those four elements of photography and practice using that new knowledge will improve your results.

LIGHT
http://photographyknowhow.com/photography-lighting/
http://photographyknowhow.com/quality-of-light-in-photography/
http://photographyknowhow.com/direction-of-light-in-photography/

COMPOSITION
http://www.photographymad.com/pages/view/10-top-photography-composition-rules

EXPOSURE
http://www.digital-photography-school.com/learning-exposure-in-digital-photography

DEPTH of FIELD
http://www.cambridgeincolour.com/...-field.htm

I agree with Edwin. Why do you want a new body? Photography is roughly 70% photographer skill/knowledge, 20% lens quality, and 10% camera body.

All of the cameras you list are entry level cameras, there isn't too much difference between them that you would notice.

ALSO… The EOS Rebel K2 does NOT use EF-S lenses, it uses the EF series. If you were to get a Canon DSLR, the EF lenses would work with it… ANY Canon EOS camera can use EF lenses, but the 35mm film, 1D, 5D, and 6D series cameras can't use the EF-S lenses (those are for the 7D and lower cameras).