I'm in the process of buying a Canon Rebel Xsi. It comes with the 18-55mm lens and the 55-200mmm f/4.5-5.6 autofocus lens. I'm wondering which of the following 2 lens' I should buy; Canon EF 50mm f/1.8 II Camera Lens OR Sigma 28-70mm f/2.8-4.0 High Speed Zoom Lens for Canon SLR Cameras? Can someone help? Thanks in advance.