Well for what it's worth. If you wanted to sell the Vivitar, it probably ain't worth much, My guess is it's worth more to you as a shooter. If you sold the Canon lense, you sell the best one you have for what? For about $300, Canon has a 100-300 zoom lens, that's less expensive, notice how I avoided saying "cheaper", than Sigma's 100-300. That and the 28-135 that came with the camera sure cover's a lot of ground! And if you just want to go walk about, the 28-300 Vivitar covers the same range but probably not as sharply, but, your only carrying one lense. There's no reason to have a $5000 lens if you have $200 technique. I got a lot of $200 lense's!