Uh what kind of upgrade are you looking for buddy? Just a tech upgrade? If your just looking for a tech upgrade and you like the feel of your D3100, then I would recommend the D3300. The 5xxx series feels smaller to me, when I had the D5100. It wasn't as comfortable in my hands as the D3100 was. But if your wanting that flip out screen, then the D5300 would be what I would recommend.
But if your looking for a more pro feel and abilities, then I'd suggest a D300 for full pro, or a D90 for great simi-pro feel! But these two choices are only if MPs aren't the priority, but great quality build,feel, and versatility of lens choices since they have a built in focus motor you can use both AF-D and AF-S using auto focus!
And if your really after the tech, but would really like the abilities of the pro like models, then I would suggest waiting and saving up for the D7100 which covers both bases pretty well from what I hear!