Hi, I'm new here, and relatively new to semi-serious amateur photography. I have a new (my first) dSLR, Canon Rebel SXi, and am so far very happy. I have only been using the kit lens, but I am now ready to buy another. I really enjoy wildlife and bird photography, so I have definitely felt the frustration of 18-55mm kit lens and have mostly kept using my old super zoom p&s for that purpose. Any lenses I buy would, at this point, be primarily for wildlife/bird photography. Right now I am struggling between buying the Sigma 50-500mm or the Canon 70-300mm with a 2x converter. It seems as though the Canon might be slightly better quality (which is saying something at this low-quality end of things) but I will surely loose quality using a telephoto zoom with a teleconverter as opposed to the single bigma. I am also concerned about the lack of IS in the bigma. Any advice would be greatly appreciated. Thank you!