Help! Which to choose this is my short list. I haven't got a super telephoto lens and it will be mated to a 60d. Mainly going to be a walk around lens, but would love to take some wildlife and some Motorsport shots. Would it be worth while getting a sigma tele-convertor if I went with the siggy to give me more reach or would I lose IQ. Seen great reviews in both lens. Oh which way to go.