The Nikon D3100 dSLR has over 40 different AF-S lenses it can use at this time. Nikon has been making professional cameras since 1959 and ones for the amateur market since 1965
Sony has only been in the business of making dSLR's since 2005 when it purchased the camera division of Minolta. The Sony A33 is a newest player,a SLT, so there not much know about it.The SLT cameras use all 19 of the alpha compliant lenses made by Sony
There have been some issues brought up here on Yahoo Answers when it comes to having Sony cameras repaired once they are out of warranty.
Any dSLR can shoot outdoors, under low light conditions and the D3100 and A33 can both shoot video. What you need for shooting under low light conditions is a fast lens.
Visit a proper camera store and hold both in your hands, especially with a lens like the 18-200 mm lens.
That will tell you more about how well you will be able to control the camera/lens combinations under the conditions you will be shooting