just purchased nmy first camera a nikon d3100 and have teh 18-55mm lens kit but looking at getting another lens for it and seen the tamron and sigma 70-300mm lens for around 180 anyien give me some feedback on what one is best

the lens i got with camera is the 18-55mm vr lens should i buy another vr lens as they seem to be alot dearer most of my shots wil be my kids or the dog running around field and some sport shots aswell hopefully

or if anyone can reccommend any other lenses that i could have a look at as im new to all this