Which lens is better for my Nikon d3100?

Http://www.henrys.com/847-NIKON-AF-S-VR-70-300-4-5-5-6G-IF-ED.aspx

or

http://www.henrys.com/61708-NIKON-AF-S-DX-55-300MM-VR-4-5-5-6G-ED.aspx

Better for what? I'm assuming you want to make far objects seem closer to you. Any will do. The 70-300mm technically should be slightly better optically due to the shorter zoom range. A non-zooming 1000mm lens would beat both three times over.

The 70-300mm has faster focusing. I suggest buying it.

They're both reputed to be very good lenses. The 55-300mm VR (I have one) is designed for DX sensors only. The 70-300mm VR is also compatible with the newer film cameras and with FX sensor cameras like the D600.It also works with DX cameras like your D3100. That extra frame coverage of the 70-300mm comes at a price in weight, bulk, and money. It is also why they gave it less range at the shorter end, because being FX increases the difficulties of design.
Both can be very sharp to the edge to about 200mm. Both lose a little bit of sharpness beyond that, but they are still very good in the center. With either, zooming to 300mm can capture a little more detail than 200mm. That differs from Nikon's cheapo version of 70-300mm, which loses more detail to blurriness than it gains from magnification. The 55-300mm VR has more softness in the corners at 300mm than the 70-300mm VR. The 55-300mm VR autofocus speed and accuracy at the longer focal lengths can be fine in bright conditions with a subject that has some good edges to detect, but otherwise it falls behind the 70-300mm VR. The 55-300mm is harder to use with a polarizer, because the front element rotates in focusing.