With more expensive models you hope to get more features. What ones do you wish you had on the 3100? For example, the D7000 has 39 focus points and the indicator can move on the diagonal. Mirror lock-up? Can you prefocus and then turn off the shutter/auto focus, so that you can release the shutter without it focusing, so that you do not miss fast action, yet still have it in focus? Is there ISO sensitivity, so minimum shutter speed and maximum ISO can be pre-set? Can you adjust white balance temperatures? How much can you post-edit photos within the camera? In the 7000 the histogram is only available AFTER the image has been taken.
The more expensive cameras are more rugged, probably less plastic. You won't find plastic parts where the lens attaches to the body. Better models should be more water resistant.
There is a learning curve trying to figure out new features or new ways of getting to features you take for granted on your camera. You will probably be saying, "I want my old camera back!" " I know how to find......."
Ask for a comparison of sensors. Take a close look at what features can be accessed or changed right from the outside of the camera body. What is the highest ISO? When does noise show up? What is the highest number aperture, f/22 or f/32?
But, binga63 makes an important point about lenses. Are the ones you have compatible. A good lens is more important than a good camera body.
PS I am not a pro.
I also noticed you have your pictures hanging in your home. When you do get a new camera consider lens focal lengths that you do no already own. You can still use your 3100 to capture shots at a certain focal length that you have only with that camera. For example you might have a good macro lens. A new camera does not automatically mean better pictures. I have seen some wonderful images taken with an iphone.