can some one tell me, i am fairly new and very green to photography, i have a Nikon D3000 with a 18-55mm and 55-200mm nikon lens, plus i have a sigma 10-20mm. i would like to know should i stick with Nikon lens? or is it ok to purchase lens like Sigma? does it matter to someone like myself that only really uses his camera on Holidays? i am looking at a 70-300mm the price difference is $350. i am not even real sure why i want to purchase another lens, maybe i should just get to understand the unit i currently have.